Who Killed Markiplier?
Short answer? It was William (the Colonel). But the full story is a little more complicated.
The Actor (Markiplier) seeks revenge against William and his ex-wife Celine after they had run off together
Mark had attempted suicide after Celine's betrayal but quickly discovered that this manor he lived in wasn't all it seemed to be when it brought him back to life numerous times.
Eventually he devised a plan to lure the Colonel down to the wine cellar when was was drunk and offers to play a game of Russian Roulette to rekindle their old friendship. This leads to William shooting Actor and killing him, therefore Actor taking over Damien's body, co-hosting it.
Actor planned to have Colonel framed for his murder and in the end, he succeeded. But not in the way he had planned (More in Damien).
Wilford Motherloving Warfstache
One of the most confusing concepts we'll see! Let's debunk it.
The full concept behind this video is summed up easily in one word. Stories.
This is explained more in depth in Damien, but these characters are in stories of their own. The entity of the manor has them all trapped in their own heads.
Abe the detective, when asked the finer details of his hunt for William, he comes up blank. He's just a character with a plot to fulfill in a world that makes no sense.
Warfstache has accepted his reality already and assumed that Abe had as well, and then has to reassure him that no, he wasn't going crazy.
This part in the storyline was meant to illustrate Warfstache's change from Colonel to Warfstache, showing his mindset and personality change drastically after what he's been through and done.
Damien
Damien, our sweet boy, look how he's massacred you. Let's talk about it !
This is the story of Darkiplier, and how he really came to be. But first, we have to establish: who, what, where, when and why?
Celine created this sort of looping purgatory for Damien to keep him safe from Actor and his plans. This obviously doesn't work out for long as Actor confronts Damien eventually.
Every night when Celine leaves the cabin, she's going out to hunt for Actor, urging Damien to sleep while she's away so the cycle can continue.
Time means next to nothing in this place. Actor is now driven by his own ego and need to be the hero, convinced that everyone else around him is the villain.
The change that Celine refers to when Damien offers to take over and help is the entity, possibly taking over them.
When the purgatory is destroyed and Damien steps out, that is his transformation into Darkiplier.
note: Markiplier TV happens directly after this but does not have much to contribute lore-wise so it will not get a separate explanation
A Heist With Markiplier
31 endings, a bunch of egos that won't be mentioned, and a sprinkle of lore! Let's go!
The basic premise of AHWM is that you and Mark are conducting a heist to steal one artifact from a high security vault, a box of some kind. From there, only chaos ensues.
There are only about three endings that contribute to lore so lets start with Darkiplier.
Dark's route directly references the original Who Killed Markiplier and features Dark making a monologue about Actor and his stories and games.
Dark alludes to the fact that Mark that led us through this heist is the same Actor that we all know.
A Date With Markiplier
A date with Markiplier, what could go wrong? How about we find out!
Much like Heist, there aren't large portions of lore in these videos, but the most stand out pieces of information are from Actor and Dark.
Dark first talks about being pushed aside, replaced and mocked (by Actor most likely).
It's also worth mentioning that in the ACCEPT ending where you accept Mark's marriage proposal, this next scene where its revealed to be only a movie or shoot. It's one of the most clear instances that we have that date/heist/actor mark are all the same person.
Plot Summary
Now for the entire plot in a few paragraphs!
Story starts off with three best friends, Damien, Mark, and William. Mark falls in love with a woman named Celine and eventually she runs off with William.
Only Damien and Mark are left and Mark attempts to kill himself in the manor he's lived in his entire life, but he's brought back to life by the malicious entity within it.
Now aware of this power that the manor holds, Mark makes a plan to fake his death by challenging William to a game of russian roulette and letting him kill him and then promptly be framed for murder, taking another body (Damien's) to use as his own.
Celine attempts to stop this but is unsuccessful and dies along of Damien (is her twin brother). She creates a purgatory to try and keep him safe but fails at this as well. With your help (the district attorney that was also there), they use your body to escape and leave you behind.
William is driven insane by watching his friends die and your former dead body get up and live, turning into Wilford Warfstache as a result of his insanity.
Damien is now Darkiplier as a result of this purgatory being destroyed and now tries to regain control from Actor, who is obsessed with being the hero in his story.