Berserker

A berserker on the party's side is hired muscle, a sworn brother from a northern raid, or a prisoner the party broke out and who now owes them a fight. The framing has to be explicit because berserkers are Neutral with Int 9, and they don't follow plans. They follow whoever paid them and whoever they trust, in that order.

In combat, point and shoot. Tell the player who's running the berserker (or the GM table next to you) that the berserker charges the nearest threat and swings the greataxe until it drops. No held actions, no flanking maneuvers, no "wait until the wizard signals." If the party wants the berserker to attack a specific enemy, they have to shout it before initiative or accept that the berserker picks their own target. The +5 to hit and 9 damage average is solid for a CR 2 ally, and once the berserker is Bloodied, that damage spikes hard with Advantage on attacks.

Bloodied Frenzy is the reason to keep the berserker alive past round two. A clever player will figure out that healing the berserker just enough to keep them in the fight, but not enough to take them out of frenzy range, is the optimal play. Let them have that moment. The berserker doesn't notice or care; they just keep swinging.

Pay them in coin or in story. Berserkers leave when the contract is up, and they remember slights for years.

A berserker is a CR 2 humanoid with a greataxe, 67 HP, and exactly one rule that matters: Bloodied Frenzy. Once you knock them under half HP, they have Advantage on attack rolls and saving throws for the rest of the fight. The encounter math flips at that moment, and the party should feel it.

Run berserkers in groups of three to five against a low-tier party. They have no ranged option and no special movement, so they walk forward at 30 ft. and swing. Greataxe is +5 to hit for 9 average slashing on a 1d12. That's a fine first-round hit, but the real threat is what happens after the cleric drops one to half. With Advantage on attack rolls, that 1d12 starts landing crits, and Advantage on saves means your party's wizard cannot reliably Hold Person them off. Don't bother with formations or flanking. These are people who picked up the biggest axe in the camp and ran at the loudest noise.

Target the spellcasters. A berserker has Int 9 and no tactical training, but it has eyes, and the wizard is the one in robes throwing fire. Have them ignore opportunity attacks to close, since the chance of dying mid-charge is worth less to them than the chance of reaching the soft target. Berserkers do not retreat. They do not surrender. The fight ends when they hit 0 or when the last of them goes down. If the party tries to take prisoners, the berserkers spit blood and attack until unconscious.

Drop the AC 13 in someone's face. These are not strategists. They are the noise that the players' good plans break against. Pair them with a single hobgoblin or bandit captain to give the players someone to actually talk to.