It's a promise made in a realm that recognises oaths when they're spoken, and that means Zerxus can feel it too. It ought to inspire hope, but that's strangled almost immediately by fear, because if that oath was forcibly broken -

He will be here when you return.

Asmodeus lies all the time, but he doesn't break a contract and the difference is more subtle than most people realise. A mortal needs to see it plainly, and literally, to accept an offer properly.

For someone whose soul is already his - there's an echo to it, a cadence that Zerxus knows as well as his own heartbeat.

Thank you.

The devil isn't doing this for no reason - there's a catch or a cost - but right now? He's willing to just accept it.

"Let's get you that food."