based on the journal pages left around dusk vigil, ser yuhelmeric went off the deep end after the calamity. his men were starving from the sudden eternal winter in coerthas, but he refused to abandon fort. there was a mutiny which he put down mercilessly which uhh... led to aforementioned cannibalism.

the dialogue from the quest-giver npc that you come across in the dusk vigil quest ("For all the nights to come") has some additional insight. an extremely corrupt inquisitor arrested his wife and confiscated the gem that was in their family for generations and presented on their wedding day. the inquisitor had it embedded in the statue of halone in dusk vigil. ser yuhelmeric wasn't allowed to take it back since it would be considered a desecration of a sacred monument. so it can be inferred that he stayed at dusk vigil to guard it until his dying breath.