This is a theory that talks about Bowser and King Boo from the "Mario" series.
Artwork of Bowser and King Boo, as appears in Luigi's Mansion
Bowser is a crazy villain. Some of his deaths at the end of Mario games are pretty brutal. An example is in Super Mario Galaxy 1, when he falls into a star and is then stranded on an exploding planet that turns into a black hole.
King Boo's role in the Mario series at first glance doesn't seem to be anything significant. He only appears in few Mario games and, aside from the Luigi's Mansion series, his role in the games he does appear in is minor.
You may think that Bowser and King Boo have nothing related to each other. However, they both play a major role together in the Mario series which keeps the series alive. While this is never explained directly, there are a few games in the Mario series which give hints about the truth of Bowser and King Boo.
Super Mario World[]
Super Mario World is the first game in the Mario series in which evidence for the theory can be seen.
King Boo was really around as early as the Super Mario Bros. 3 days in which Boos first appeared, he just never appeared in-game.
Have you realized how occasionally there were these "Ghost House" levels in Super Mario World? These houses are where Boos lived peacefully in their afterlife. Then came Mario, who stormed into theses houses and wrecked havoc numerous times throughout the events of the game, often with malicious intent. The Boos knew they had to stop this, so they reported Mario's actions to King Boo. King Boo was furious about this, so he started tracking down Mario.
Similar levels of havoc would continue in future Mario games, thus beginning the Boo's and King Boo's hatred toward Mario that continue to this day.
Paper Mario[]
Paper Mario is the second Mario game that provides evidence for the theory.
The main attention in this game for the theory goes to Bowser, but King Boo deserves to be mentioned as well. This is because Mario had actually allied with the Boos in World 3 of this game, and so you may think this would change King Boo's mood about Mario. However, if you think more into it, you'll see that this brief alliance wasn't nearly enough to make up for Mario's past aggression. First, he only allied with one Ghost House, compared to the several he disrupted in past games. Second, the alliance was forced, because they wouldn't give him the Star Spirit until he helped solve their problem.
All in all, what Mario did in Paper Mario wasn't enough to stop King Boo's hatred toward him.
Bowser plays a much bigger role to the theory in Paper Mario. The end of the game is where it gets particularly interesting.
This starts in the scene where Mario, Bowser and Kammy Koopa are on a platform way up in the clouds that is about to explode. Mario is seen escaping with the help of the Star Spirits. Bowser on the other hand isn't so lucky. Explosions can be both seen and heard without him or Kammy Koopa escaping.
While Bowser's deaths before this game were at least understandable on how he survived (maybe he can swim in lava and only got injured in the smaller falls), there is no explanation on how Bowser can survive being stranded on a exploding platform thousands of feet in the sky, no matter how tough or resilient he has been in the past. Mario fell from the same height at the very beginning of the game, and even he got killed, though he was fortunately revived by the Star Spirits.
Layout of where the "Toad Town News" sign is located. Notice the sign on the left.
In fact, proof about Bowser's death may be hinted in-game. At the very end of the game when you must walk to Peach's Castle to trigger the ending scene, go to the center part in town.
In front of Merlin's house should be a sign, which is called the: Toad Town News. After mentioning about how Mario defeated Bowser and how Peach and her castle are safe, it also mentions this:
Bowser was badly beaten and will likely never return.
We're sure of it.
Almost positive.
After saying the obvious on how Bowser was badly beaten, it also states on how confident they are in that Bowser won't return. This connections that can be made with this series of events strongly suggest that Bowser had suffered such a brutal beating that he had finally been killed.
But wait, if Bowser is "dead," then why is he still alive as the main villain in the Mario series today? This is because the theory still expands to the Mario game released after Paper Mario; Luigi's Mansion.
Luigi's Mansion[]
Luigi's Mansion is perhaps the most important and significant game to this theory.
You all know what happens at the start of the game. Luigi goes to a mansion that he "won in a contest" only to find out that it is much darker than advertised, and needs to find out where Mario is, who went into the mansion before him.
This game features King Boo's first in-game appearance; he appears after Luigi opens a trapdoor and a bunch of Boos and King Boo fly out of it.
About mid-way through the game, you see King Boo in a fancy room with Mario inside a painting, struggling to get out. This is the first sign that King Boo is up to something sinister with Mario.
But, soon after, perhaps the most crucial event in relation to the theory occurs. In Area 3, do you remember the Fortune Telling Ghost? Also known as Madame Clairvoya. You know, the kind ghost who tells Luigi to bring Mario's dropped objects so she can see what happened to Mario?
Well, once you give her the last object, after she tells the orb to show them all it can about Mario, she will say this:
Wh-what is this!?
Bowser?!
How can this be?! I see the hideous form of Bowser!
Is Bowser somewhere in this mansion? I cannot believe it...
And yet I see it...
I thought that Mario had soundly defeated Bowser!
Has King Boo somehow revived Bowser? This could be horrible! Well, for you.
While she confirms that Bowser still exists, she doesn't understand why considering Mario soundly defeated Bowser. Remember though that Paper Mario was the Mario game before Luigi's Mansion and how the Toad Town News sign in Paper Mario also thought that Bowser wouldn't return. Also recall Bowser's brutal defeat at the end of Paper Mario. See a connection here?
Also note the part when she mentions how King Boo could have revived Bowser. This will be important later on.
Another connection can be made when Luigi opens the door to King Boo's room. Before the final battle begins, King Boo talks to Mario's painting, and at one point says this interesting line:
I remember how much trouble you've (Mario) caused me in the past.
This doesn't make sense at first, considering this is King Boo's first appearance in the Mario series, but remember what I said earlier about how Mario kept on going into Ghost Houses in Super Mario World? That and other confrontations with Boos is what King Boo meant by the "trouble."
King Boo then turns the Mario painting into a Bowser painting, then goes into the painting and sucks Luigi in. You will then fight the final boss on the roof of the mansion; Bowser with King Boo controlling him.
Now, remember how the Fortune Telling Ghost mentioned on how King Boo could've somehow revived Bowser? The connections here imply that King Boo wanted to get revenge on Mario for the trouble he caused with other Boos by first luring him into a mansion filled with ghosts, then reviving his now dead arch-enemy; Bowser.
After Luigi's Mansion[]
The evidence doesn't stop after Luigi's Mansion. If you pay attention, you will see that Bowser's deaths after Luigi's Mansions are more violent and extreme then before.
The previously mentioned Super Mario Galaxy 1 is a good example of this. After his defeat in the final battle, Bowser falls into a star. This should surely burn him to death, as even if he can swim in lava the gravity should flatten him. Yet he appears later on a planet that is getting destroyed. The planet then explodes and turns into a black hole, all with Bowser still stranded on it.
It may seem like there is no way Bowser can return from that and other brutal defeats in recent Mario games. But there may actually be one explanation.
Just recently I explained how King Boo revived Bowser. Well, even after his apparent defeat by Luigi at the end of the game King Boo didn't get completely destroyed in Luigi's Mansion. Why? He is a ghost, and as you all know, because ghosts are already dead they can't die. What King Boo did instead, though, is give Bowser some help. He took over his body to possess him and keep him alive, believing Bowser to be the best vessel to find a way to get revenge on Mario and make his life miserable.
You know the tales on how ghosts come for revenge and haunt you forever? This is exactly what is happening to Mario.
This theory can explain some other oddities with Bowser in more recent Mario games, such as his sudden shift to a very sinister personality. In most games before Luigi's Mansion, Bowser's plan is mostly to just capture Peach and keep Mario from saving her. In games after Luigi's Mansion, Bowser focuses on trying to really marry Peach, actually killing Mario, and to conquer the Mushroom Kingdom. Sometimes he even takes it to another level to conquer the Universe (like what was seen in Super Mario Galaxy).
This sudden darker shift proves only further that Bowser is controlled by King Boo. With Bowser now sharing the same sinister personality King Boo did in his in-game appearances, it would make sense that Bowser's plans would get more extreme as well. And why would this King Boo-controlled Bowser still try to marry Peach? Not necessarily because King Boo is in love with Peach, but because he wants to break Mario's heart.
This is also why Bowser keeps coming back, no matter how brutal his defeat is. Since he is controlled by King Boo he won't die as he is now part ghost. And of course, the fact that this part-ghost Bowser would never truly die only emboldens him to have more extreme plans.
Further Proof[]
Now, one may think some source of magic, mainly from Kamek, may be reviving Bowser alive instead. But it's hard to imagine this is the case in some of Bowser's recent deaths. In both Super Mario Galaxy games, Bowser's final defeat is getting sucked into a Black Hole, and it is hard to imagine anyone going in with him to revive him, lest they be killed too.
Only King Boo, a ghost who can't die, can help Bowser somehow find a way to survive still intact. Perhaps he even made another full, physical copy of Bowser's body, something no character has been proven to do in the Mario series, other than King Boo at the end of Luigi's Mansion.
In some of the more recent Mario games, such as New Super Mario Bros. for the Nintendo DS, a new form of Bowser with only his bones called Dry Bowser appears. This form appears after Bowser had just fallen into lava following his defeat. It seemingly makes no sense for Bowser to have his skin burnt off by the lava and still function perfectly with only his bones.
But this theory can provide sense into this situation. While the lava would damage Bowser's body itself and burn all the skin off, it wouldn't impact King Boo's possession of the body. This would allow Bowser to function fine with only his bones.
In fact, Bowser's carcass is actually not the only body which is possessed after it is defeated. Dry Bones enemies, which are the remains of regular Koopas, are able to still function and be a threat to Mario despite being dead because regular Boos are possessing them. Since King Boo is more powerful than the other Boos, it would make sense for him to be able to possess the King of the Koopas; Bowser. As a more powerful Boo, King Boo may also have the unique ability to return Bowser's skin later on as well.
After Luigi's Mansion, King Boo himself only physically appeared in Mario spin-off games such as Mario Party and Mario Kart, as well as a select few other Mario games such as Super Mario Sunshine and Super Princess Peach.
King Boo's appearance in these games may seem to debunk this theory, but pay attention to how King Boo looks in the Luigi's Mansion games, and how he looks in all other games. The Luigi's Mansion King Boo has features unique to all other Boos such as the glowing eyes, purple tongue, and the crystal crown. The King Boo in all other Mario games do not have any of these features. He more resembles Big Boo in these games instead; just a larger version of regular Boos, except with a crown on his head.
What this most likely indicates is that the King Boo you see in Mario Kart, Mario Party, and other non-Luigi's Mansion games is just either a fake or replacement king while the true King Boo is busy controlling Bowser.
King Boo as he appears in Luigi's Mansion: Dark Moon
Luigi's Mansion: Dark Moon[]
In Luigi's Mansion: Dark Moon, King Boo, likely frustrated from a series of defeats while in possession of Bowser, escapes from Bowser's body to try to come up with a plan while in his ghost form, perhaps trying to take a different approach than usual in order to achieve success (Bowser doesn't make an appearance in the game at all, which can also be seen as evidence for the theory).
It should be noted that King Boo never made it clear that he managed to escape his painting he appeared to be trapped in at the end of the first Luigi's Mansion game, in order to make sure Mario couldn't connect him to Bowser. However, he saw Professor E. Gadd's decision to sell the painting in this game as a good opportunity to make it appear as though he escaped the painting after being trapped in it.
Despite King Boo's efforts to get revenge on the Mario Bros. and achieve domination by shattering the Dark Moon, Luigi again manages to defeat him in ghost form. King Boo of course doesn't really die at the end of the game. He just returns to Bowser's body to think of his next plan.
Super Mario 3D World[]
The plot of Super Mario 3D World may provide proof to the theory. In this game, instead of capturing Peach, the King Boo-controlled Bowser instead attempts to kidnap the seven Sprixie Princesses. This decision may appear to make no sense from Bowser's perspective, but with the knowledge of this theory it actually does. The reason is that the Sprixies are the only ones who can actually destroy ghosts. This leads to King Boo kidnapping the Princesses instead of Peach, as he identified these Princesses as a true threat to him. Although defeated, King Boo was still able to escape.
Another Theory[]
Another similar theory to the one explained above is that King Boo is actually Bowser's ghost. The theory would have mostly the same concept, except this time, there was no true King Boo before Paper Mario, and when Bowser died in Paper Mario, his spirit became King Boo, which was first seen directly afterwards in none other than the next game chronologically; Luigi's Mansion. Therefore, the trouble in the past that King Boo meant in Luigi's Mansion would not specifically be for the trouble he caused in Ghost Houses, but the trouble he caused him while he was alive trying to capture Peach.
King Boo then built and revived his own body, which, with the exception of Luigi's Mansion: Dark Moon, used to try to get revenge on the Mario Bros. over and over but with failure each time. With the fact that Dry Bones are referred to as dead Koopa Troopas possessed by their own spirit in the form of Boos, it makes sense that Bowser, the king of the Koopa Troopas, would be possessed by his own spirit in the form of King Boo, and Bowser also having a Dry Bones-like form with Dry Bowser.
This could also better explain how Bowser, even after Luigi's Mansion, still showed an infatuation toward Peach. Since under this theory King Boo is the dead form of Bowser, it would make sense that he would still try to marry Peach, and it would also make sense that he take it more seriously and go a step forward with extreme and dangerous plans of world domination now that he doesn't need to worry about the fear of death since he is now, in fact, undead.