Well, antiisepticeye is jacksepticeye's alter evil ego. He's a demon who's trying to take control of jack, but jack always manages to fight him and regain control. Anti has canonically "killed" Jack twice; once by the slit of the throat and the other killing him with a disease.
Darkiplier is Markiplier's alter evil ego, and he's trying to gain control too. Dark is a little more complicated; he is a dark presence inside of marks body who is also possessed by a male and female spirit. Dark is angry at Mark because mark promised to let him in again but he didn't, so dark shows up from time to time. Now, there's also a lot of complicated lore about Dark being connected to the fabric of space and time, but that's kind of hard to get into. Jack and mark are best friends in real life, so dark and anti have an uneasy aliance.
