Idol of Endurance

The exiled cards return to your graveyard immediately after Idol of Endurance leaves the battlefield. Any cards that had left exile before then (most likely because they were cast with Idol of Endurance's second ability) remain in their current zone.

If a spell has X generic mana in its mana cost, you must choose 0 as the value of X when casting it without paying its mana cost.

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 the card has any mandatory additional costs, those must be paid to cast the spell.