Cultist Fanatic

A cultist fanatic on the party's side is a heretic, a defector, or a useful idiot. Maybe a god the party serves has flipped one of the cell to their cause. Maybe the fanatic believes the party is fulfilling a prophecy and will follow them until the prophecy ends, at which point they expect to be sacrificed. Sell the conditional nature of the alliance up front, because a fanatic with a different patron is a betrayal waiting for a trigger.

In combat the fanatic supports rather than tanks. Spiritual Weapon as a bonus action gives the party a free 1d8 force per turn at any target the fanatic can see within 60 feet. Command 2/Day works just as well on enemies as on PCs, so spend one early on a melee threat ("flee") and save the second for the boss. Hold Person on a humanoid antagonist sets up the rogue for an entire round of crits. Pact Blade only comes out when the fanatic is cornered.

Out of combat, the fanatic is a problem. They will try to convert NPCs, deface shrines to other gods, and argue with the cleric. They believe their interpretation is correct and the rest of the party is provisionally tolerated. Set clear lines for what the party will not allow them to do, and have the fanatic test those lines weekly.

Pick the doctrine the fanatic will not bend on. When the party finally asks them to bend on it, they leave or they fight.

A cultist fanatic is the priest of a cell, not a footsoldier. Run them as the voice of the encounter, the one who keeps shouting scripture while regular cultists bleed for them. Mechanically they are a half-caster with a Pact Blade, which means they can hit at melee range, project a Spiritual Weapon at distance, and disable a key PC with Hold Person or Command. That is enough levers to matter at CR 2 if you sequence them well.

Open turn one with Spiritual Weapon as a bonus action, summoning the spectral force somewhere the party cannot easily ignore, then cast Command on the front-line tank. "Drop" or "flee" both buy a round of repositioning for the rest of the cell. Turn two, lead with Hold Person on the heaviest single threat, then Pact Blade. The Pact Blade hits at +4 for 6 slashing plus 7 necrotic, and a paralyzed target eats auto-crits from any ally inside 5 feet. The fanatic should be standing next to the orcs, not in front of them.

After Hold Person is spent, the fanatic falls back on Command 2/Day plus Spiritual Weapon as a bonus action every round it is up. Spiritual Weapon does not require concentration, so it stays even if the fanatic gets hit. Concentration on Hold Person, however, is fragile at AC 13 and 44 HP. The party will break it. Plan for the fight to last three or four rounds, not eight.

A fanatic does not surrender. The conviction is the whole character. At low HP they should be screaming a final invocation, swinging the Pact Blade at whoever silenced their spell, and refusing healing the party offers in exchange for information. If the party wants the cell's secrets, they need to take prisoners earlier.

Give the fanatic a name and a single repeated line of doctrine. The party should remember the words, not the stat block.