I don't think it's that OP.
From the looks of it, the bigger the enemy, the more HP they have, the longer you have to channel it to get the effect off. And, if you get interrupted, you lose the Death effect and only get the very minor damage over time it does. Plus, EVERY enemy in that trailer immediately targeted Noctis when he started using it on them, in an attempt to counter him. And quite a few got some hits it and interrupted the effect.
Several, like the Iron Giant and the Bandersnatch, were too powerful for him to just Death right out of the gate, so he ran around, tossing other abilities or letting his buddies beat the critters down, until their health got low enough for him to get the full channel off that he needed to activate the effect.
I think it works. I means that there is risk/reward to using it. If you can channel it over time, you'll get to instantly kill a foe, and regain a little bit of HP. But, you're basically defenseless while doing it, and if you get hit, there were WAY more efficient ways to be spending your time.
Without knowing how long the full fights against some of those foes were (there were clear cuts in the Bandersnatch fight), and without knowing how long some of those enemies take to kill without Death, it seems way to early to call it OP. I mean, for basic Goblins and such, they're never much of a barricade to Final Fantasy adventurers anyway.
Yeah, I more or less agree with Skyblade. You're super vulnerable during the attack; it seems very easy to interrupt. For those who've played ZoE2, it makes me think of the Vector Cannon , albeit more practical.
Returners Represent!