Slytherin Common Room

(When this card enters the battlefield pay its build cost
and attach it to a land you control or sacrifice it. That
land is indestructible. This card's abilities may only be
activated when that land becomes tapped)
Build: Tap an untapped black creature
you control
{t}: Add {b}, if this mana is used to cast a
Slytherin creature: give target creature -1/-1