Official Riftbound ruling

What happens when you play Abandon on a spell that was played for its Flow cost?

It will be banished when it attempts to leave the chain.

“[T]hen banish it” in flow’s text is short for “if it would leave the chain after becoming a finalized chain item, and leaving the chain wasn’t instructed by its own execution, banish it instead.” Abandon returns the countered card to hand after countering it, but for the purposes of flow’s “then banish it”, this is also leaving the chain. No matter how it happens, as long as the flow spell leaving the chain wasn’t instructed by the execution of the spell itself, it will be banished.

This also applies to effects like Kai’Sa, Evolutionary from Origins, or Fizz, Trickster from Spiritforged.

Riot's question and answer, word for word, from theVendetta Rules FAQ and Clarifications

The cards

  • Abandon
    Abandon

    Spell

    Reaction (Play any time, even before spells and abilities resolve.)

    Counter a spell. Return it to its owner's hand instead of putting it in their trash.

    Predict. (Look at the top card of your Main Deck. You may recycle it.)

  • Kai'Sa, Evolutionary
    Kai'Sa, Evolutionary

    Unit

    Ganking (I can move from battlefield to battlefield.)

    When I conquer, you may play a spell from your trash with Energy cost less than your points without paying its Energy cost. Then recycle it. (You must still pay its Power cost.)

  • Fizz, Trickster
    Fizz, Trickster

    Unit

    When you play me, you may play a spell from your trash with Energy cost no more than 3 Energy3 Energy, ignoring its Energy cost. Then recycle it. (You must still pay its Power cost.)

More rulings on these cards