Fort Revenant

If Fort Revenant would enter, sacrifice an untapped Mountain instead. If you do, put
Fort Revenant onto the battlefield. If you
don't, put it into its owner's graveyard.
{t}: Add {c}{r}.
{1}, {t}: Fort Revenant deals 1 damage to target attacking creature.
"It's completely impenetrable. The only way to bring this thing down would be to sneak inside and transform into a dragon or something."-Gomery, Halfling Barbarian