Septiplier is the name of a "ship" created by certain portions of the Jacksepticeye and Markiplier fanbases. As the name probably implies, Septiplier is the concept of Jack and Mark being in a romantic relationship.
Both Jack and Mark have expressed their distaste for the pairing. Most members of both their communities have stopped pushing it.
Nowadays, the only people still pushing for Septiplier are most likely doing it as a joke or for attention.
Septiplier as a Character Edit
In the video "Markiplier TV" by Markiplier, at the very end a character named "Septiplier" is partially shown. When this character speaks, they speak with both Mark and Jack's voices simultaneously.
This character also seems to have both Jack and Mark's arms, as they are the only parts we see of them.
They seem to be a combination of Jack and Mark that wishes only for death, which they are granted by Wilford Warfstache at the end of Markiplier TV.
It is also possible that this "Septiplier" is not a character at all, and rather a representation of Septiplier as a concept.
This character was a jab by Markiplier and Jacksepticeye at the concept of Septiplier, as well as away of saying that it is doomed to die.