The Hollow Sage is an optional boss located at a building far right of Mallrus.
The Hollow Sage is a large bloody, reanimated mummy. The multilated body parts on the floor suggests that he is akin to Frankenstein; made up of different dead body parts. The player will witness him come back to life thanks to his necromancers.
The fight against the Hollow Sage may prove to be the lengthiest battle in the whole game. It is advised to go against this walking corpse and his unending army of satanic medics with a friend or two.
The Hollow Sage is not a very difficult foe; he moves exactly like the Gory Mound. What's important is to take down every Satanist that appears on screen. Or so long as you can keep his backup busy, that would work. The Sage does possess the ability to heal himself, but not nearly as much as the Satanists, whom will heal the Hollow Sage dramatically in less than a few seconds; whacking at the mummy and ignoring the Satanists will keep the battle going endlessly.
First thing you should do is kill just three of the Satanists, making sure one stays alive. No more Stanists will spawn in if there's just one left. This will keep the Hollow Sage's health regen at a minimum, making it possible to wear him down faster than he can gain health back. If you kill all four Stanists, more will just spawn in.