Onslaught | Magic: The Gathering
Formats Onslaught is Legal in
Official Oracle Text for Onslaught
Official Rulings for Onslaught
10/4/2004 : The ability is not optional. If you have the only targetable creatures, you have to tap one of your own.
4/1/2008 : A "creature spell" is any spell with the type Creature, even if it has other types such as Artifact or Enchantment. Older cards of type Summon are also Creature spells.