Beseech the Mirror

Bargain represents an optional additional cost. A spell cast with that additional cost paid is "bargained."

Some instant and sorcery spells require additional targets if they're bargained. You ignore those targeting requirements if those spells aren't bargained, and you can't bargain those spells unless you can choose the appropriate targets. On the other hand, you can bargain a permanent spell even if you won't be able to choose targets for an enters-the-battlefield ability of that permanent once the spell resolves.

Bargain means "As an additional cost to cast this spell, you may sacrifice an artifact, enchantment, or token."