Standstill

When a player casts a spell, sacrifice this enchantment. If you do, each of that player's opponents draws three cards.

Mystery Booster 2

When a player casts a spell, sacrifice Standstill. If you do, each of that player's opponents draws three cards.

"You learn a lot in no time when you're about to get your neck snapped."
—Dack Fayden

IDW Comics Inserts

Odyssey

When a player plays a spell, sacrifice Standstill. If you do, each of that player's opponents draws three cards.

It only works once. If someone casts another spell after it triggers, but before that trigger resolves, then it will trigger again. The first time one of these triggers resolves, it will be sacrificed for the full effect. Any additional triggers on the stack will do nothing when they resolve because you will be unable to sacrifice it additional times.