Goblin Nightstalker

Goblin Nightstalker is unblockable if target
opponent controls a Swamp or Black creature
{r}: Do 1 damage to any creature attacking this
turn. Use this only if they have taken damage
this turn from a creature source.
{b}: If a opponent has a artifact in play, gain
control of it and place a thief token on it.
Once it has three thief tokens on it, return it
to it's owner's control.