Grave Sentinel

Cumulative upkeep: add {b} to your mana pool.
Mana added to your mana pool this way doesn't
empty from your mana pool as steps and phases
end; then at the beginning of your end step, if
{b}{b}{b}{b}{b} was added to your mana pool this
way, exile Grave Sentinel, then return it to the battlefield transformed under its owners control.