Giant Wolf Spider

A spider on the party's side requires a hook, and the cleanest one is a druid or ranger PC who has befriended the local pack. The spider is unaligned and operates on instinct, but Animal Friendship, a high Animal Handling check, or sustained feeding can produce something that follows the party for a session or two. It is not a pet. It is a wild predator that has decided this group is part of its loose hunting territory.

Use the spider as a scout. With Stealth +7, climb 40 ft., 60 ft. darkvision, and 10 ft. blindsight, it is a near-perfect dungeon advance party. Send it ahead of the formation, let it climb up to a vantage, and have the druid relay what it sees through Speak With Animals or shared sense magic. In combat the spider's job is opportunistic strikes against creatures the party has already engaged. With 11 HP and no listed attack actions in this stat block, it will not survive a direct exchange, so use it for flanking and to draw a single attack of opportunity per round away from a wounded PC.

The relationship has a clock. The first time the party leaves the spider's territory, it does not follow. The first time food is scarce, it eats one of the horses. Plan for the parting and make it specific.

Have the spider have a name only the druid uses. The rest of the party will hate hearing it, which is the point.

A giant wolf spider is an ambush scout, not a swarm bug. It does not spin a hunting web. It runs targets down on 40 ft. of walk, climbs walls and ceilings at 40 ft., and sees in pitch dark out to 60 feet plus 10 ft. of blindsight if the lights are out entirely. With Stealth +7 against a Passive Perception of 13 and the spider sitting on the ceiling, the party will not know it is in the room until it drops on someone.

Lead with positioning. Place the spider above the party from session prep, not from initiative. A good opening turn is the spider dropping from a 30-foot ceiling onto the back-line caster's square, no fall damage to itself thanks to climb speed and the descent being in increments. The spider then sprints around the room using climb to stay out of reach of the front-line, dragging the engagement toward whoever has the lowest AC. Eleven HP and AC 13 mean it will not survive a focused round, so every turn that the spider is not adjacent to a target is wasted.

Run them in pairs or threes against a low-level party. One spider is an inconvenience, two is a real fight, three with terrain is a session-defining moment because the party suddenly has to fight in three dimensions. Giant wolf spiders are unaligned beasts, so they will absolutely break off and flee if a friend is killed and the math looks bad. Have them retreat into a tunnel mouth or up into the rafters, and if the party does not pursue, the spiders will be back the next night while watch is set.

Treat them as a pacing tool more than a threat. The CR 1/4 stat block is what the GM uses to remind the party that the dungeon has things in it that hunt at night, and that bedrolls in the open have consequences.

Cut the torches in the room before the spider drops. Players who fight at disadvantage from blind, against a creature that sees them just fine, learn to bring better light next session.