Can only be used by hunters in the Beast Mastery spec. Minimum level 39 in Shadowlands.
You must learn the skill to tame Gargons, which is taught by a Gargon Training Manual, a guaranteed drop from Huntmaster Petrus, a rare NPC that can appear in the Witherfall Ruin area of south-east Revendreth. It appears this manual can only be read at level 60.
Reza, and its companion Geza are pets of the rare NPC Huntmaster Petrus and can appear in the Witherfall Ruin area of south-east Revendreth. They can only be tamed after Petrus is defeated. They must be present and still alive when you defeat Petrus and you must tame them immediately afterwards, or they won't be tameable. Note that sometimes they're buggy and the tameable version doesn't appear after he's defeated.
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.