UngeheuerLich
Legend
its a full round action in 3.X to coup de grace (if you don´t invest a feat) and you can´t ready a full round action.
A standard action which instantly kills sounds just wrong. So max damage is more reasonable. If you have time however, you could just say you can prepare the death stroke for as many rounds as you wish, doing max damage times the number of rounds you spent preparing and apliciate it all together in one stroke. No save, no insta kills. Maybe you may have a saving throw to be only unconscious and not dead, bleeding for X per round.
A standard action which instantly kills sounds just wrong. So max damage is more reasonable. If you have time however, you could just say you can prepare the death stroke for as many rounds as you wish, doing max damage times the number of rounds you spent preparing and apliciate it all together in one stroke. No save, no insta kills. Maybe you may have a saving throw to be only unconscious and not dead, bleeding for X per round.