Whenever Kain deals combat damage to a player, that player gains control of Kain. If they do, you draw that many cards, create that many tapped Treasure tokens, then lose that much life.
Cast cheap, evasive (Jump = flying on your turn) Kain, pump its power, and connect for big swings that draw you cards equal to the damage and net you a pile of Treasure — paying for it with life and temporary loss of the creature. You then claw Kain back (it returns to the command zone when it dies, or via theft/return effects) and repeat, snowballing card advantage and mana to outvalue the table before closing with the resources you've banked.
Engine-level card draw and Treasure generation that scales with Kain's power, refueling the whole deck
Only 3 mana with built-in evasion, so it gets value fast and rebuilds cheaply after dying
Treasures fix mono-black's color/ramp needs and enable big X-spells and combo payoffs
Commander recursion: dying Kain heads to the command zone, so the gift is rarely permanent
You willingly hand a (possibly large) creature to an opponent — disastrous if you can't reclaim it
Life payment equal to damage punishes greedy pumps and rewards opponents who race you
Mono-black means no clean answers to enchantments or some artifacts
If opponents simply keep Kain and don't attack you with it, you've ramped your foe
Legendary Creature — Human Knight
