Darkiplier

Darkiplier is one of Mark's prominent characters, having been featured in various live action videos and sketch comedies on his YouTube channel. Originally being a collective fan name for Mark's dark persona that was featured in his early work, Mark later took the idea behind the character and made him into a primary antagonist of Who Killed Markiplier?. Darkiplier is also a recurring character in Mark's one-off videos.

Appearance
Aside from Mark's regular appearance, Darkiplier dons a monochrome suit. His silhouette is often grayed out and accompanied by a dark foreboding aura that causes his appearance to become visually distorted. At times, Darkiplier's eyes will turn pitch black without any visible pupils.

Who Killed Markiplier?
It is revealed that Darkiplier is actually a fragmented fusion of Damien, Celine, and a mysterious Entity (held inside Markiplier Manor) mixed together in the body of the Protagonist.

The series started when Mark, out of jealousy for the Colonel's love affair with ex-wife Celine (Damien's sister), planned to frame the Colonel for his “death” by a game of Russian Roulette, tricking him into thinking he wanted to mend their broken friendship and by the end of it would be able to steal the Colonel's body. The reason for this is actually because the Manor itself is somehow connected to another dimension where the laws of physics/nature don't apply: the Upside-Down.

In the Upside-Down, there are these "ideas/entities" that are able to whisper into your ear and one of them is connected to the house, making the house itself a character. Due to Mark living in the house longer than anyone, The House is able to whisper ideas to Mark and convinces Mark that these ideas were his own: get revenge on the Colonel.