Feldon of the Third Path does not have summoning sickness.
Thornbite Staff attached to Feldon of the Third Path.
Each of your opponents has at least one card in hand.

Activate Feldon of the Third Path by paying and tapping it, creating a token copy of Diamond Lion with haste.
Activate Diamond Lion by tapping it, discarding your hand, and sacrificing it, adding .
Thornbite Staff and Stalking Vengeance both trigger.
Resolve the Vicious Shadows trigger, dealing damage to target player equal to the number of cards in their hand.
Resolve the Thornbite Staff trigger, untapping Feldon of the Third Path.
Repeat.