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.
Spawned during quest Adversarial Blood. 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.
Level scaling: In Shadowlands 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.