Autophage skills are unlocked during "The Giant-Slayer" main story quest. Autophage skills are very powerful as they grant your slayer extra stat boosts. You can upgrade your slayer's Authophage Tier by equipping multiple Autophage Skills. At Tier 3, you will gain a substantial boost to your health and damage, but this will come at the cost of no Health Recovery and limited health regeneration from health items.