Reanimation spell a flesher casts on its host's skeleton, creating the symbiote that becomes a war mage.
In Examine's account of War Mages, Boned is the pivotal step. After a Flesher consumes a living being and melts the original skin away to re-sleeve it, the flesher casts Boned, reanimating the skeleton. The two then become a symbiote.
Symbiotes are prime targets for other fleshers, and repeated re-sleeving strengthens the being each time — until, after roughly twelve to sixteen iterations, the magic becomes sapient and skeleton, flesher, and magic fuse into a single war mage.
The reanimation step that binds flesher and skeleton into a symbiote.