Quiet Anger (3.5e Trait)
From D&D Wiki
You are bottled within yourself and letting no emotions visible, but within is a mix of everything that has been kept inside.
Benefit: You gain a +4 to bluff and when letting your emotions go, you gain temporary "Die Hard" feat.
Drawback: −4 to Diplomacy and Fortitude saves
Roleplaying Ideas: A character with this trait may be any class with a temper problem and it would work well with a Barbarian.