Reminds me of my Darigaaz's Command (which was a spin on Darigaaz's Charm). It's like a supersized Leeching Bite/Steal Strength, or indeed like Steal Strength+Consume Strength. (If an efreet steals your strength then consumes it, that makes the efreet roar? Um.)

I was envisioning more of a war cry, or even howl where the efreet is hyping themself for battle (the buff) and the shout is so loud that it hurts their enemies (the damage).

> Target creature gets +/+ UEOT, then ~ (or that creature) deals damage to all other creatures

?

Maybe

> Creatures you control get +1/+1 and gain trample UEOT
> ~ deals some damage to creatures you don't control

Trample btw is nice here since the damage already dealt to blockers this turn counts toward the damage "going over" to face.

I think the design seems fine. It's quite potent, but I might expect to see it at uncommon. I can see it has potential for a blowout, and might be rare for that reason, but it looks like a Khans of Tarkir style rare - the kind of disappointing "Why isn't this uncommon" kind of rare.

Eh, I did think about costing it at but worried that might be too big a swing at 3 mana.

If it was uncommon, you'd probably need it to be a sorcery, too.

This needs to be "can't be countered by spells or abilities" as still needs to fizzle without legal targets.

Giving trample on your opponent's turn is useless so there's that. IMO the flavor is lessened if the war cry is performed on opponent's turn (maybe even on their end step).