Relentless( Unearthed Arcana, p. 90)
You don't know the meaning of the word "tired." You go all out until you simply can't continue.
You gain a +2 bonus on Constitution checks and similar checks made to continue tiring activities (see the Endurance feat, page 93 of the Player's Handbook, for a mention of all the checks and saves to which this benefi t applies). Drawback: Any effect or condition that would normally cause you to become fatigued instead causes you to become exhausted. Roleplaying Ideas: Characters with this trait may see others as soft or weak, especially anyone who complains about being tired or fatigued. They might openly scoff at others' weaknesses or might quietly encourage them to "tough it out."