Giant Spider

A giant spider on the party's side is a druid wild shape, a drow envoy's mount, a familiar bonded to a warlock with the Great Old One patron, or a temple-spider tended by a clergy that worships in caves. Pick one and let the handler do the talking. The spider has Int 2 and no language, so the ally framing always sits on whoever is holding the leash.

Use the climb speed. A scout-spider can navigate a tower from the outside, a chimney from the inside, or a flooded crypt by walking the ceiling above the water. Stealth +7 plus Darkvision 60 ft. plus a 30 ft. climb makes the spider the most efficient infiltration animal in the bestiary at this CR. It cannot pick locks or read maps, but it can cross a guard chamber that no humanoid scout would attempt and report back what it saw, in whatever halting way the handler interprets.

In combat the spider is a one-attack disruption. Drop it from a ceiling onto an enemy spellcaster, bite once, and accept that the 26 HP is a downpayment on a missed Concentration check. It will not survive a counter-round of focused attacks. The handler should treat it like an expensive scroll: useful once per scene, then gone. If the party is fond of it, give them a chance to retrieve the body and revivify the bond rather than just rolling up another beast.

The spider will not improvise. If the handler is dropped or unreachable, it climbs to the highest point in the room and waits. It will not retreat to safety on its own, and it will not attack on instinct. It simply stops being useful.

A giant spider is a ceiling problem. The 30 ft. climb speed is the only stat that matters for encounter design, because the moment you remember to put the spider on the wall the fight gets meaningfully harder for melee characters and trivially easy for the wizard with a crossbow. Pick your battlefield: a webbed cavern with a 40-foot ceiling, a barn loft with rafters, a forest with a thick canopy. If the spider is at floor level the encounter is a beat. If it drops from above into the marching order, it's a scene.

Open with Passive Perception 14 catching the party first. Stealth +7 plus Darkvision 60 ft. means the spider has been watching for at least a round before initiative. Have it pick the trailing party member, descend on a silk strand, and bite once before anyone else acts. With AC 14 and 26 HP the spider folds to one focused round, so its job is to land that opening hit and force the party to reorganize their marching order on the spot.

In open combat at floor level the spider is an Int 2 beast that bites the closest moving thing. It has no tactical sense and no instinct to break grapples or reposition once it has chosen prey. A clever party can pin it against a wall and drop it in two rounds. The interesting fight is the one in three dimensions, where the spider scuttles up to the rafters between turns and forces the fighter to either climb or just stand there.

If the spider drops below 13 HP and is on a webbed surface, it retreats up its own silk to a hole in the ceiling and hides. Beasts don't fight to the death over territory. They eat what they catch, and a giant spider that catches nothing today eats next week.

Hang a wrapped cocoon from the ceiling somewhere visible. Don't say what's inside it. The party will figure it out.