The Whomp Mushroom is a special Mushroom that changes Mario into Whomp Mario, that allow him to crush enemies by falling over, and making him invincible to certain attacks, like small Fireballs, or an Iceball. Mario will lose his Whomp form if a powerful enemy hurts him.
The Whomp Mushroom is quite similar to the Thwomp Mushroom, that turn Mario into Thwomp Mario. It is however, completely grey. His eyes and mouth are like those of a Whomp. It also has grey eyebrows and two plasters, like a Whomp.