Fighter / BarbarianMulticlass Build Guide — D&D 5e

A fighter already has the most attacks in 5e — two at 5th, three at 11th, four at 20th — and Action Surge doubles them for one round. Rage adds flat bonus damage to every single one of those Strength melee hits (+2 rising to +3 at Barbarian 3), and resistance to bludgeoning, piercing and slashing halves most incoming damage. The multiplication is what makes this build: six attacks in an Action Surge round, each carrying rage bonus, behind damage resistance that effectively doubles your hit points.

Barbarian 3 is the breakpoint because it unlocks a Primal Path. Bear Totem extends resistance to all damage types except psychic while raging, turning an already durable fighter into the single hardest thing to kill in melee. Alternatively, Zealot adds free extra damage every turn with no resource cost. After 3 you stop — the fighter’s third and fourth attacks at 11 and 20 are worth far more than anything barbarian levels 4+ offer.

The cost is real: you cannot wear heavy armour while raging (you lose Fast Movement and the rage’s feel, though technically you can), and you delay your third attack and your ability score improvements. The build works best as a two-handed Strength fighter who was already planning to swing a greatsword — Rage does nothing for archers or Dexterity builds.

Ability priority: STR → CON → DEX — Strength drives both your attack rolls and rage damage. Constitution is hit points and concentration-free durability. Dexterity is largely irrelevant — you are in medium armour at best while raging.