Skeleton Mastery is a very important skill if you have any intention of using skeletons to help your Necromancer make it through the game. Plus it will scare the bejesus out of all. This skill will dramatically improve the life, band damage of Skeletons, Skeleton Mage, and revived monsters.
History
As the Necromancer's knowledge of the undead improves, he is able to harness and divert more powerful spirits to inhabit the corpses of the recently dead. A Necromancer who possesses this skill is able to raise skeletons and skeletal mages of a much more powerful variety.