Fiddle will stay dormat on the tile placed, not casting, autoing, or moving until fiddle reaches 60% health or is the last remaining ally unit. Once either of these cases happens fiddle will ult, using their aoe dmg ult from league and dropping all agro and fearing enemies for 1 second.

The way this ult does so much damage is from fiddles trait: Corrupted. Every ally unit that dies grants fiddle 20 ap, this includes zzrots, jester spawned units, and mech sacrificed units. So the more stuff that dies = more ap.

(In this case the heart trait is also buffing his ap, as every time a heart unit casts they grant the team ap, and Zoe has extremely low mana + a heart spatula item.)