Black Shadow

A mysterious existence, that gives the impression of a jellyfish out of water.
Normal attacks have no effect on it, so its appearance is a portent of doom. There is nothing to do but wait to be engulfed and erased.
Just when you think you've got a handle on how things work after the Saber and Rin routes, this thing shows up as a surprise enemy out of nowhere.
It was a favorite of the chief director. In fact, he loved it so much that he actually tweaked the event CG where it first shows up and used it as his desktop wallpaper.

Fate/side material: Fate Encyclopedia