From Diablo Wiki
The necromancer class in Diablo II could obtain an itemset named "Trang-Oul's Avatar". Upon completion of the set the necromancer would transform into a vampire in appearance. Additionally he would obtain a number of skills such as Meteor, Fire Wall and Fire Ball. The set was designed for the necromancer, however any class could wear the items in the set except for Trang-Oul's Wing. Therefore only the necromancer could complete the set.