Who is Caliban? He's a mutant with the ability to sense other mutants within a 25 mile radius, and as well as being a Morlock, Caliban has been a member of X-Factor and X-Force. However, it's his relationship with Apocalypse which will no doubt come into play in. As a Morlock, the character looked pretty emaciated and unimpressive, but he later transformed into a far more impressive figure when he decided to align himself with Apocalypse.According to ComicVine , "For a while Caliban was known as Hellhound, but was later officially announced as Death, Archangel's replacement in Apocalypse's Horsemen. He would use his enhanced senses and strength to locate more mutants for Apocalypse, and his personal anger and rage to punish his enemies." How much of this will factor into X-Men: Apocalypse remains to be seen, especially as Singer doesn't always pay that much attention to the source material!