"Death By Witch Bolt" is a recurring gag in which Freezo murders unfortunate foes, animals, passersby and even allies with the first level Warlock spell Witch Bolt. This is sometimes an attempt to gain temporary hit points, which are provided by the "Dark One's Blessing" whenever he kills a creature.
Freezo unleashed his first Witch Bolt during The Audition (1.01) to commit the senseless murder of Leonard Swiftfeatures. He attempted to use it again shortly after to kill Barry Swiftfeatures in It Like She Me (1.02), but was thwarted by his shackles, which prevented him from performing the spell's somatic components.
Witch Bolt is a horrible way to die: graphic and painful, it involves melting and boiling flesh.