Stirge

A stirge on the party's side is a niche call. The most plausible framings are a swamp-witch's familiar, a druid's wild-shape novelty, or a single trained stirge a hag NPC has lent the party for one job. With 5 HP, AC 13, and a 40 ft. fly speed, it is a precision tool, not a combatant. Use it once per scene and put it away.

The job is the Proboscis attack at +5 for 6 average piercing on the hit, then 5 average necrotic at the start of each of its turns while attached. Sent at a single sleeping or distracted enemy, that's a quiet way to bleed a target out over a minute without alerting the rest of the room. Excellent for assassinations the party doesn't want traced back to them. Less excellent in open combat, where the stirge will be slapped off or killed by the first incidental swing.

The other use is reconnaissance. A 40 ft. fly speed and a Tiny size lets the stirge slip through arrow slits, chimneys, and broken windows the rest of the party can't approach. The player who controls it gets to scout one room at the cost of risking the bug, which is a fair trade. Pair it with a sending spell or a familiar-style shared sense if the campaign supports it; otherwise, the stirge comes back and the player describes what it saw.

The stirge will not bond with the party. The moment it's full or threatened, it leaves. Treat it like a leech in a jar: useful for one thing, kept at arm's length, and replaced when it's gone.

A single stirge is a joke. A swarm of stirges in a dark room is one of the most stressful encounters in the SRD, and that's the version you should run. Eight to twelve stirges in a cave roost is a CR 3 to 4 encounter that feels like a CR 8 because the party cannot easily target them, cannot easily peel them off each other, and cannot rest until every one is dead.

Open in the dark. Stirges have Darkvision 60 ft. and the party often does not. The first sign of trouble is the sound of wings and the first proboscis hit at +5 for 6 piercing, followed by the stirge attaching. While attached, the stirge can't make further Proboscis attacks but the target takes 5 average necrotic at the start of each stirge turn. Multiply that by three or four stirges latched to one PC and the wizard is bleeding 15 to 20 HP per round with no ability to cast somatic spells. The PC or an adjacent ally can detach a stirge as an action, which forces a brutal trade-off: spend the action peeling, or spend the action killing the rest of the swarm.

Target priority is the lowest-HP creature in the party. Stirges are not smart, but they swarm low-Con prey instinctively. With 5 HP and AC 13 each, a single hit from any party member kills one, which means area damage is the answer and the party should reach for it fast. The complication is that an attached stirge takes the same damage as the PC if the area effect catches both, so reckless burning hands costs the wizard half their HP before the cleric finishes screaming.

Stirges do not retreat. They feed until they're full or dead. A full stirge detaches and flies off, which means about 10 HP of feeding and then it leaves. The party who survives the first encounter with a stirge swarm will start carrying torches into every cave. That is correct.