[Snake]

Ether Invitation at red HP doesn't do enough damage for the restrictions, mostly because how Defense and Armor works right now. It's pretty laughable imo, and nobody uses it anymore.
You lose HP, hoping to deal tons of damage, but then do less than a normal hit.

Just make it unresistable like Spooks said.

(But, if you're more daring, make it two separate procs like Narcus. One for the basic hit, then another for the unresistable Ether Invitation damage. And if the attack gets Stalemated/Eviter'd, the damage is halved or nullified.
And then, of course, longer Unmarkable cooldowns (like 4-5 rounds) in exchange.)