Desperate to avenge the slaughter of his people, Caliban became colder and harder during his brief stint with X-Factor. After a battle against the Horsemen of Apocalypse, Caliban became increasingly frustrated with his passive power’s inability to grant him the revenge he sought. When X-Factor subsequently encountered Apocalypse, Caliban swore to serve him as his Hellhound in exchange for power enough to exact revenge. Apocalypse genetically altered Caliban to increase his size and strength. After Caliban proved his worth against Archangel, Sabretooth, and Cable’s New Mutants, Apocalypse granted Caliban a place in his Horsemen as Death. With the Horsemen, Caliban fought the X-Men, but was defeated, unaware that the Apocalypse he and his fellow Horsemen were serving at the time was Mr. Sinister in disguise.
Now separated from Apocalypse’s influence, Caliban’s intelligence began diminishing. He kidnapped Jubilee, holding her hostage in exchange for Sabretooth, an ex-Marauder whom Xavier was trying to rehabilitate. Kitty Pryde, now Shadowcat, accompanied Sabretooth into the Morlock tunnels. There, she was left to die by the villain after they encountered a giant subterranean squid. After rescuing Pryde, Caliban faced Sabretooth in a brief but ferocious fight, which resulted in his face being scarred, and he fled. Later, Caliban was hunted by the Dark Riders, Apocalypse’s replacements for the Horsemen. Saved by Cable, Domino, and Storm, Caliban helped them to track the Riders back to their base in Egypt and confront their new leader, Genesis. As his intelligence continued to degrade and he began reverting to his original, nonviolent personality, Caliban was recruited by Cable into X-Force. With that team, Caliban faced opponents such as Mimic, Barrachus the Kalinator, Sabretooth, Holocaust, the Externals, S.H.I.E.L.D., Mr. Sinister, an other-worldly foursome, the alien android Pulse, Mojo, and Asgardian rock trolls and goblins. For a brief time, Caliban and X-Force fell under the control of Sebastian Shaw, Black King of the Hellfire Club, and were mentally coerced into attacking Cable.
Caliban started having crippling seizures that couldn’t be explained. After aiding Cable against an assault on the Xavier Institute by Bastion’s Operation: Zero Tolerance forces, Caliban was taken by Ozymandias, the immortal indentured servant of Apocalypse, who revealed that the seizures were a result of Caliban’s past alteration. When he returned to Apocalypse, Caliban took on the form of the Horseman Pestilence. At Apocalypse’s behest, Caliban successfully captured both Cable and his alternate reality counterpart Nate Grey. All the Horsemen were bested by the X-Men when Mikhail Rasputin teleported them to safety. Subsequently, Caliban tracked down Cyclops, who had merged with Apocalypse. During a period when Apocalypse’s psyche became dominant, he released Caliban from his service. By that time, Caliban had once more begun reverting to his original naïve self, and his intelligence faded until he possessed only a simple, animal-like mind.