After The Party reached the shop (travelling aboveground), he corpsplosioned them again, then after they entered the shop he used a form of magic missile augmented with vile damage, a form of damage so evil that it could only be healed while on consecrated ground. Angel and LittleOne took him down in a single round, but with the aid of Baron Deathmoor, he formed a new, 12-foot, body from pieces of zombie, with damage reduction and a massively improved AC.
Corpsplosion is similar to fireball, but dealing negative energy damage instead of fire damage and with a higher save DC. It also requires a nearby corpse to blow up, although ambulatory corpses (i.e. the undead) can also be used. This destroys the corpse.