Unprintable in its current incarnation, as you can use it to gain control of the creature that's blocking it. Tempted to make this a prowl-y Sower of Temptation instead, something like:

1. When ~ ETB, if an opponent was dealt combat damage this turn, gain control of target creature that player controls for as long as you control ~.
2. The first time ~ deals combat damage to a player, gain control of target creature that player controls for as long as you control ~.

But I don't know how black either of this is. The second option is also a bit ambiguous when it comes to having more than one opponent.

­Conquering Manticore in black. Lifeloss enough to offset color pie bending?

Before:

Infestor
Creature - Zerg [r]
When ~ enters the battlefield, gain control of target creature an opponent controls until end of turn. Untap that creature. It gains haste until end of turn. That creature's owner loses 2 life.
2/2

• Old version has too much text, happens too suddenly, and acts too much like a lava axe for our liking.
• is too much mana to do anything with the other creature.
• New card based on a limited Sower of Temptation, and plays much more fun.