Grim Trenches

At the beginning of your upkeep, you may
add {b}{b}{b} and, until end of turn, you
don't lose this mana as steps and phases
end. If you do, lands you control can't be
tapped for mana this turn.
{5}{b}: Until end of turn, Grim Trenches
becomes a 5/5 black Elemental creature
with deathtouch.