I’m no Great Moustache, but I’ll try to respond in his place. Advantage targets the attribute dice, adding an exception in the game to target the d20 rather than the attribute dice would likely be confusing. Instead, I would suggest reducing guard as Great Moustache proposed. As for amount, I’d say do the inverse of Resistance Boon without the immune part. So tier 1 is -3 Guard. Tier 2 is -6. And tier 3 is -9. This essentially makes it so you gain 3/6/9 damage on an average roll (assuming you normally beat the roll by 3).
Now for auto-max damage, since OL doesn’t have damage dice separate from the attack roll, there is no max. You could, theoretically, roll ten thousand damage. The odds of that are very close to zero, but technically possible. Instead… I propose the following for what you want:
Ambush (Tier I-III)
Cost: 3 Feat Points
- Tier I: Agility 4
- Tier II: Agility 5
- Tier III: Agility 6
A favorite of assassins, highwaymen, hunters, and others who attack from surprise. You are skilled at finding the perfect places to strike when the target isn’t aware of you.
When a Damaging Attack you make at an enemy who is not aware of your presence, reduce their guard by three for each tier of this feat to a max of -9. If you kill the target, you do not reveal your presence to others and the kill is silent.
Cost: 3 Feat Points
- Tier I: Ambush III feat and Agility 7
You have so excelled at stealth combat that opponents vitals are more obvious to you and leaving oneself vulnerable to your attacks could spell death instantly.
When making a Damaging Attack at a creature with ten Guard or less after reduction by Ambush and they are not aware of your presence, the attack counts as a Final Blow. For targets with higher guard but still not aware of your presence, if your attack deals half the targets max health, the attack counts as a Final Blow.
The reason for this is that it allows you to kill lesser creatures as by the time you get Assassinate, most enemies you will face will have too high of guard to just outright kill, but by dealing half their HP in one attack, you can still assassinate them. Bosses are immune to Final Blows until their HP equals 0 as well, preventing one shotting the boss. However, Assassinate is still quite powerful, but the cost of needing to be level 7 and a total of 12 feat points into the build make it late game and something the GM can build around if he doesn’t want certain enemies assassinated.