The Liche King
In ages past, powerful sorcerers learned how to cheat Hades and persist as undead liches long after death had robbed their bodies of life. A dangerous practice of Theurgists is to bind the souls of these ancient liches to their will, thus acquiring their powers.
The Liche king is arguably the best pet in the game. He is especially useful due to his ability to inflict damage from a distance.
His Unique Skills: Death Nova (emits a wave of death that will deplete life from any living being it contacts), Wraith Shell (allows the Liche King to diminish its corporeal presence making it less vulnerable to all types of damage) and Arcane Blast (drawing upon ancient arcane craft the Liche King forms deadly bolts of elemental energy and launches them at enemies).