The Ur-Sphinx
Eminence — As long as The Ur-Sphinx is in the command zone or on the battlefield, other Sphinx spells you cast cost {1} less to cast. Flying Whenever one or more Sphinxes you control attack, each player mills that many cards. For each player, you may cast a card that player milled this way without paying its mana cost.
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Official rulings (2) · Scryfall
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.