Giant Centipede

An allied giant centipede is a druid's choice or a swamp-witch's attendant. Speak with Animals, animal companion features, find familiar adjacent rituals: any of these can put a centipede on the party's side for a scene. The framing that holds best is "the witch in the hut sent her pet ahead to lead us to the shrine", because the centipede has no combat value worth scheming over and the lead-the-way angle uses its actual strengths.

In play, the centipede is a scout and a key. Climb 30 ft. plus Blindsight 30 ft. means it slips under doors, up walls, through grates the party can't fit through. Describe what it senses back through the witch's link: vibrations, body heat at the edge of its range, the shape of a room without sight. With Int 1 it cannot relay anything complex, so the party gets impressions, not reports. That ambiguity is the fun.

If a fight breaks out around the centipede, get it off the field fast. AC 14 and 9 HP mean a single AOE ends it. Have it scuttle into a wall crack at the first hostile action and reappear at the next quiet moment. The party should feel responsible for it the way they would for a candle in a wind.

Give the centipede a name the witch uses with affection. Players who hear "Pebbles is going first" stop treating it as a stat block and start treating it as a member of the group.

A giant centipede is a 9-HP atmosphere creature. It exists to remind the party that the cave they just lit a torch in is alive at the floor level, not to threaten anyone past level 2. The stat block is almost defiantly minimal: no actions list, no traits, no special senses beyond Blindsight 30 ft., Int 1, Wis 7, AC 14. It crawls, it bites if you give it a bite, it dies in one hit from a real character.

Run it in numbers and from above. Climb 30 ft. is the entire reason this monster gets used: a centipede on the ceiling drops onto a passing wizard and skitters across them, and a centipede in a sleeping bag found by hand is worse than any combat encounter you could write. With only one PB-2 attack profile and no listed actions in the SRD entry, treat any bite as the GM's adjudication: a small piercing hit, maybe a Constitution save against poisonous secretions if your table likes that flavor. Don't roll initiative for one centipede. Roll it for six.

The encounter shape is short. With 9 HP and AC 14, even a single first-level fighter folds the swarm in two rounds. Make the rounds count. Describe the centipede that just bit the rogue trying to crawl into their pack. Have one wrap around the cleric's lantern and crack the glass. Use Blindsight 30 ft. to keep them coming out of darkness even after a Light spell goes up. The fight should feel like an infestation, not a battle.

Place the centipedes before the party can see them. Spider-style ceiling drops, a rustle in the bedroll, the dry click of legs on stone behind a barrel. The dice rolls are nothing; the body horror is everything.