ToME: the Tales of Maj'Eyal

I would like to make an addon that, when you get damaged, checks if you had full health, and if you did, then if you would have died before you get a move it sets your HP to 1 instead and gives you a move. what thing should I modify to do this?

So as a passive, applies to all characters thing? Or is this a talent?
We need a bit more detail here. Also have a look at how the Second Life talent works.

I meant it to be an addon applied to all characters to prevent deaths i view as unfair. Forinstance, rogue attacks a dragon with full hp. Rogue gets one shot. it prevents that type of thing. But I don't really understand how to use a hook to do this

You could give every player the flat_damage_cap stat that Ghoul has.