Forsaken hunters automatically know how to tame undead creatures. Hunters of other races must learn the skill from a Simple Tome of Bone-Binding which is a rare drop from miscellaneous Elite NPCs throughout Maldraxxus and its associated dungeons: Plaguefall and the Theater of Pain.
Can be spawned through the quest Adversarial Blood in Skettis. When he's first summoned, Vakkiz appears as a living wind serpent (with a different ID) who cannot be tamed. He transforms into his tameable, undead form half way through the fight. Unlocking this quest requires completing most of the Skettis quest chain and farming for a number of summoning items. See the quest link for detailed information.
Level scaling: Most NPCs will scale with the Hunter's level, within the constraints of their level range. Hunters can tame regular NPCs up to 2 levels higher than them, but can only tame elite NPCs of the hunter's level or below.