Trang’Oul is a dragon-god, hence some of the items being named after parts of his body (wings, claws, scales). He is meant to be responsible for maintaining “the balance” of Sanctuary and tasked first his disciple Rathma, the Nephalem and then Rathma’s progeny (the Necromancers) in helping him with his task.
Lore-wise, it’s unknown why the wearer turns into a Vampire, but considering it’s a Necromancer Item Set, a vampire is certainly thematic.
Trang-Oul’s Avatar is designed for a Necromancer, with a class-specific Shield (Totem), Body Armor, Helmet, Belt and Gloves. This set overall gives many good bonuses.
Patch 1.10 added a nifty bonus to wearing this full set: the character transforms into a Vampire.
* For the sorceress these skills are capped at +3 as they are native to that class.
