The scream makes their ears ring and pairs with their own when his resolve digs the knife into their hand just before it falls. Iago kicks out wildly, blindly, until they feel their foot connect with it and send it flying across the floor. Pointless, they know, -of course they know, this is all pointless, all of it- considering the endless fount of blades he has, but their frightened, fried brain will gladly take one less weapon at his disposal.
Physically, they're no match for him, it isn't worth the effort to try to stop him from jerking them around like a rottweiler with a toy. They're about to choke out a spell to hold him when he throws them to the floor and knocks the wind from their lungs, cutting them off with a garbled cry.
Their nails scratch up the skin at his hands around their throat, and they meet his eyes again, nauseated by the charred, bloody mess they've made of one. A flake of burnt flesh sloughs off onto their cheek below and they scream the best they can without being able to take a full breath.
Iago can't stand to hear him talk, can't stand the way he keeps trying to shatter their illusion of their brother there just below the surface. They fight like a cornered animal, like a furious child with no courtesy to play fair. Thrashing, they manage to stomp harshly on his tail, grinding their heel into it. Their claws leave his shredded hands to grab for his face instead, finding purchase on his earring - made of their very own tooth - and ripping the piercing with a vicious yank.
The words spilling out of his mouth make them feel small and stupid, and then even smaller and stupider since they know they're intended to do just that. Yes, Iago ran away. Yes, they dragged their brother with them. Yes, they've ended up in this wretched, cold house, no better than before, with the delusion that it would help. No, no, it wouldn't help, they know, they know, nothing ever helps, but they couldn't take another minute surrounded by hungry eyes and drenched in blood and cloaked in white and waking up on the ornate altar due to a newfound habit of sleepwalking. They know now that it's a matter of blood and destiny. They know they can't hide, they know they can't be safe or happy or free or alive for much longer, but they couldn't take it, they couldn't stand even just one more prayer -
Desperate scrambling for a makeshift weapon lands their hand on something heavy and solid - a bookend from the mantle, they'll note later on, but presently have no time for identification. They bash it down onto his forearm with all the force they can muster until they hear the crack of bone. Even then, they have only a moment where his hand loosens just enough from their bloodied throat for them to choke out another spell, throwing him backwards. Crying out to Puck hasn't proved helpful yet, fine, so the next coughing rasp of words is an attempt at aiming Hold Monster through black-edged vision and blood dripping into one eye.