Quasit

A quasit on the party's side means a warlock has bound it as a familiar, an exorcist has it on a leash before sending it back to the Abyss, or someone in the party made a deal they shouldn't have. The quasit is Chaotic Evil and resentful regardless. Run it as a snarky cat that knows things and refuses to share them unless asked precisely.

In combat, the quasit's contribution is information and harassment, not damage. Open with Invisibility and scout the next room rather than swing. If it does engage, Rend at +5 with the Poisoned rider is genuinely useful for setting up the rogue's flank, since Poisoned applies to the target's attacks and checks. Scare once per day, used on the boss caster the moment they roll initiative, can buy a clean turn off a hard fight. Shape-Shift is a utility tool: fit through a centipede-sized gap, swim a flooded crypt as a toad, or fly a message in bat form.

The cost of the familiar is roleplay. The quasit steals from the rogue, lies about what's in the next room, and pushes the warlock toward whatever the worse moral choice happens to be that night. It remembers every promise the warlock made when summoning it. If the party banishes the quasit, it goes home and reports who is responsible, which is a slow-burn campaign hook the warlock player will love or hate.

Have the quasit name itself. Whatever name it picks, never let the player rename it. The thing decides what it is called.

A quasit is a CR 1 demon with 25 HP and AC 13, and it is in your campaign because it works for someone. Quasits do not freelance. There is a warlock, a cultist, or a junior abyssal lord behind it, and the encounter pays off when the party realizes the quasit is a messenger or a saboteur, not a brawler. Speak Common and Abyssal, sneak in (Stealth +5), set something up, and either flee or die obstructing the cleanup.

Open with Invisibility before initiative if the party hasn't already spotted it. The quasit's job in round one is to set up a flank or land a single Scare. Scare is DC 10 Wisdom on one creature within 20 feet, Frightened on a fail, repeating saves at end of turn, automatic success after one minute. DC 10 is low, but Magic Resistance does not help the party here, and the disadvantage on attacks from a Frightened wizard is worth a turn of trade. Once Scare is spent (1/Day), Rend at +5 for 1d4+3 slashing also imposes the Poisoned condition until the start of the quasit's next turn. Stack Poisoned on the rogue and the rogue's sneak attack vanishes.

Resistance to cold, fire, and lightning, plus immunity to poison damage and the Poisoned condition, means the wizard's Burning Hands is largely a waste. Force damage, radiant, and silvered weapons are the answers. Shape-Shift gives the quasit 40 ft. of fly in bat form, which is the escape button. As soon as it's Bloodied, it shape-shifts to bat and is gone through a window.

The damage is forgettable. The unsettling part is the quasit speaking the cultist's words back to the party in the cultist's voice three sessions later. Treat the encounter as the introduction of a recurring familiar, not a one-off enemy.

Have the quasit giggle when it takes damage. Players hate that, which is the right reaction.