There's a floor tile that can be opened in the same room in which you encounter Herbertius (it's over on the right-hand side of the room). Inside you'll find a cursed heart, and you just need to go into your inventory and break it which causes Herbertius to die. The tactic I found worked best was to draw the skeletons away from Herbertius (they move faster than he does) and take them out, then run back into the room to get the heart and destroy it (trying to grab it when both Herbertius and the skeletons are all attacking you tends to be a bit too hectic). Also, when I fought Herbertius he was able to permanently damage my skills, so keep an eye on that and try to avoid getting hit in melee by him.