Billy, Flayer's Thrall

Lifelink
Whenever Billy reduces a creature to 0
toughness, you may pay {b}{b} and tap a spirit or
zombie you control with converted mana cost 4
or greater. If you do, regenerate the 0 toughness creature under your control.
Creatures under your control that you do not
own have Billy's abilities and "Sacrifice this
creature: Amass 1."
"Try not to move. It'll be over soon."