Trained Berserker (3.5e Feat)
From D&D Wiki
Trained Berserker [Fighter, Multiclass]
You bulk up and hone your skills at the same time.
Prerequisite: Fighter Level 1st, Rage
Benefit: Your fighter and barbarian levels stack for the purpose of determining your rage ability. Your fighter and Barbarian levels also stack for the purpose of qualifying for feats that require a minimum fighter level. Your fighter and barbarian levels do not count each other when determining experience penalties from multiclassing. Example: Krothoc a 4th-level fighter/7th-level barbarian has greater rage and can rage 3 times per day as if he were an 11th-level barbarian and would meet the prerequisites for Greater Weapon Focus. Krothoc also would not take experience penalties from multiclassing unless he took a level in another base class.
Special: A fighter may select Trained Beserker as one of his fighter bonus feats.