A support commander equipped with a bolt pistol and a Power Sword. They can be upgraded with a Jump pack and many Spells. The Chief Librarian can be produced in the Sacred Artifact.
Despite the name, Librarians aren't here to punish you for overdue library books, instead they provide psychic support, casting spells that can heal your men, buff their speed, buff their morale and damage your enemies and disable structures.
It's a good idea to keep your Librarian protected as they aren't super in melee.