Onslaught

Oracle Text

Whenever you cast a creature spell, tap target creature.

Card Rulings

10/4/2004 Can target an already tapped creature.
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.

Additional Information