Die Ur-Sphinx

If a Sphinx you control enters attacking, it won't cause the Ur-Sphinx's last ability to trigger.

You cast the spells while The Ur-Sphinx's last ability is resolving and still on the stack. You can't wait to cast them later in the turn. Timing restrictions based on the cards' types are ignored. If you cast a spell "without paying its mana cost," you can't choose to cast it for any alternative costs. You can, however, pay additional costs. If a spell has any mandatory additional costs, you must pay those costs to cast that spell. If a spell has {X} in its mana cost, you must choose 0 as the value of X.