I noticed that Retribution skill does not work with shadows in the forbidden pit on hirge. They're both undead and outsiders, aren't they, so she should get at least the normal amount. Nothing happens instead.
I did some testing, it seems like Hirge's retribution skill is totally broken.
My cleric get 1 mana from killing shadows. But Hirge doesn't get any. I also went to deadwood and tested Hirge with skeletons and ghosts, but she still doesn't get any mana points from killing them. Same goes for Imps in Lannegar Mine.