What does Irisalis's Floral Descent do?

The clone is the part that matters, because Irisalis's whole kit is built around fighting alongside her clones. The skill also appears under the name Flora Fall, where the clone it leaves is called an Iris Clone. It is the same move: area damage on landing, one clone left on the field.

In practice you open with it. The usual sequence is to create a clone first, then spam dash with Whirling Blossom Rain while the clone fights with you. Her dash fires multiple homing petal projectiles, so the 30 seconds of clone time is the window where that spam is doing the most work.

Floral Descent is not her only way to make a clone. Her core skill also summons one, and its name is not settled in the game: it appears as Iris Shadow, a clone that lasts 30 seconds, and as Whirling Blossom, which spawns up to two shadow clones at a time when it hits. Her Floral Storm is a separate spinning attack that can hit up to three targets at once.

Not settled: Floral Descent also appears as Flora Fall, and the clone it leaves as the Iris Clone. The name is not fixed across the game yet.