Angel of Highlander
You're immortal. Well, almost immortal. Instead of just taking hits and loose health, you have 10% chance of just dropping dead on hit. Place your bets.(Stealed from Crimsonland)
Not harder that an usual game, just remember that any enemy caries deadly danger.
I'll just pinch this idea quickly.
Angel of Highlander
You're immortal! Well, okay. Not quite immortal. You don't have a health bar or anything, but every time somebody hits you, you have a 1% chance to die. Getting hit increases this chance, too! But it's okay, you can heal to reduce the chance.
Getting hit by an enemy: Increases chance by 1-5%, depending on strength
Hit by nuke: Increases chance by 100%, then hits you again. This hit will kill you if it passes the chance.
Small heal: Reduces chance by 5%
Large heal/Berserk/Invincibility: Reduces chance by 25%
Supercharge/Megasphere: Reduces chance down to 1%
Armour passively decreases the chance by 2% times armour value. If this decreases it to a negative number, you're invincibile until it goes back to 1% or higher.
EDIT: Mm. I don't really like the idea of suddenly dying on 1% chance due to unluckiness.
Addendum to Angel of Highlander:
If you get hit and it passes the chance, your character goes "Oh damn it, that hurt!" and it increases the chance by 11-15% instead of 1-5%.
You can't die, but if the chance increases beyond 100%, the game finishes because you're too badly injured to continue.