When one of the Zenith spells (e.g., Black Sun’s Zenith or Green Sun’s Zenith) from Mirrodin Besieged resolves, it is shuffled into its owner’s library instead of going to the graveyard. While there aren’t many, there are a few ways to give spells Flashback. Past in Flame and Snapcaster Mage are recent examples of cards with effects that grant Flashback to other cards. When a spell that has been played via Flashback resolves, it is exiled. What happens when these two interact?

One portion of the Flashback effect is that any card that is played via a Flashback ability is exiled when it would leave the stack. This is applied regardless of why the Flashback-ed card would leave the stack, such as if it resolves or if it is countered. It also applies regardless of where this card would go. Most often, instants and sorceries go to the graveyard as the last part of their resolution. However, the Zenith tries to shuffle itself into its owner’s library before that; but the effect from Flashback will step in and change this. So, in the end, the Zenith will be exiled and the library won’t be shuffled at all.