Undead swamp

Undead swamp enters the battle field
TAPped. At any time you may UNTAP this land buy paying {p/b}.
{p/b}{t}: either summon a zombie token 2/2 or
add {b}{b} to your mana pool.
"the corpses help this dark oak to grow tall and
give this place the shade i like." Haber Bosch.