Drider

A drider on the party's side is a stretch, but it has one strong frame: a drow exile cast out of Lolth's grace and trying to undo what was done to her. The drider speaks Elvish and Undercommon, so direct conversation is on the table, and the Chaotic Evil alignment is a problem the party can either ignore for the duration of one job or treat as the slow corruption of an uneasy ally. Pick which version you are running and tell the players in the opening scene.

Use the drider as a scout and ambush partner, not a frontline fighter. The 120 ft. Darkvision and +10 Stealth and Spider Climb make her the best lookout the party will ever have in caves and ruined keeps. In combat she should hang on the ceiling, drop Faerie Fire on the chokepoint the party is trying to clear, and use Web (DC 14) to pin enemies the rogue wants flanked. Three forelegs at +7 are nice damage, but her real value is the spell trio plus the position you cannot otherwise reach.

She will not enter sunlight willingly. Sunlight Sensitivity gives her Disadvantage on everything outdoors during the day, and any plan that requires daylight work is one she sits out. Plan night raids, dungeon work, and indoor jobs around her, and accept that half of the campaign map is closed to her.

The exit is bleak. A drider does not become a drow again. When the job is done, she goes back to whatever lonely corner of the Underdark she came from, and the party should feel the weight of having used something that cannot be saved.

A drider fights from the ceiling. Spider Climb plus a 30 ft. climb speed plus 120 ft. Darkvision plus +10 Stealth means the players' first sign of trouble should be an arrow of poison snapping out of the dark from somewhere overhead. AC 19 and 123 HP at CR 6 make the drider durable, but the real lever is the Magic of the Spider Queen bonus action: Darkness, Faerie Fire, or Web at DC 14, no components, with Web Walker letting the drider ignore its own webs while the party slogs through them.

Open with a Poison Burst from 60 to 80 ft. up. The +6 attack roll for 13 average poison damage at 120 ft. range is your sniper opener, and it tells the party they have a problem they cannot reach with melee. On the second turn drop Faerie Fire on the cluster, then Multiattack with three Forelegs (or two Forelegs and a Poison Burst) on whoever is now visible and lit. Faerie Fire grants Advantage on the drider's own attacks, which makes the +7 reach 10 ft. forelegs land for 13 average each. Save Web for when the party tries to climb up after the drider, since they will be Restrained on a failed DC 14 Dex save and the drider can keep shooting from above the web.

Sunlight Sensitivity is the difficulty knob. Indoors, in a deep cave, in the Underdark, the drider runs at full power. In daylight or under a Daylight spell, the drider has Disadvantage on every attack and most of its checks, and the encounter softens dramatically. Pick the lighting on purpose.

Driders do not surrender, but they do retreat. Once bloodied, the drider drops Darkness on its own square and uses the climb speed to pull back along the ceiling to a chokepoint where it can ambush again. It will fight to the death only if the party threatens whatever it has been set to guard. Otherwise it lives to come back two scenes later, more bitter and better positioned.

Telegraph the drider with a webbed corpse two rooms early, eyes still wet. The players should know what is hunting them before they meet it.