Barbarian / FighterMulticlass Build Guide — D&D 5e

Reckless Attack gives you advantage on every Strength melee attack for an entire turn, and rage adds flat bonus damage to each hit. Action Surge then doubles your attack count for one round — at barbarian 5 with Extra Attack, that is four reckless, raging swings in a single turn, each with advantage and +2 rage damage. No other one-level investment produces that kind of spike on a barbarian.

Fighter 1 alone is already worth it. A Fighting Style (Great Weapon Fighting to reroll low damage dice, or Defense for +1 AC) is something the barbarian class never offers, and Second Wind gives you a bonus-action self-heal that does not cost a rage or a ki point. Constitution save proficiency from Fighter 1 is a bonus, though barbarians already have it — so most builds start Barbarian 1 for the d12 hit die, then decide later when to dip.

Fighter 2 adds Action Surge. Two levels is the entire cost. You lose one rage use and delay Relentless Rage at barbarian 11 by two levels, but the nova turn Action Surge creates — especially with Great Weapon Master online — is worth more than the 12th and 13th barbarian levels were ever going to give you.

Ability priority: STR → CON → DEX — Strength is attack rolls, damage and the stat rage cares about. Constitution is hit points, Unarmored Defense and Relentless Rage saves. Dexterity is the other half of Unarmored Defense but stops at 14.