Aranea

Shapechanger. The aranea can use its action to polymorph into a Medium humanoid, or back into its true form, which is a Large spider with two humanoid arms and the face of a woman. Its statistics, other than its size and AC, are the same in each form. Any equipment it is wearing or carrying is transformed along with it. It reverts back to its true form if it dies.

Spellcasting. The aranea is a 5th-level spellcaster. Its spellcasting ability is intelligence (spell save DC 14, +6 to hit with spell attacks). The aranea has the following wizard spells prepared:

Cantrips (at will): acid splash, dancing lights, minor illusion, shocking grasp.
1st level (4 slots): charm person, mage armor*, sleep
2nd level (3 slots): misty step, suggestion, web
3rd level (2 slots): dispel magic, major image

*The aranea casts these spells on itself before combat. 

Spider Climb. The aranea can climb difficult surfaces, including upside down on ceilings, without needing to make an ability check.

Web Sense. While in contact with a web, the aranea knows the exact location of any other creature in contact with the same web.

Web Walker. The aranea ignores movement restrictions caused by webbing.

Multiattack. In humanoid form, the Aranea makes two claw attacks and one bite attack. In spider form, it can make one web attack in place of a claw attack.

Claw. Melee Weapon Attack: +6 to hit, reach 5 ft., one creature. Hit: 9 (2d6+3) slashing damage.

Bite. Melee Weapon Attack: +6 to hit, reach 5 ft., one creature. Hit: 7 (1d8+3) piercing damage, and the target must make a DC 13 Constitution saving throw, taking 21 (6d6) poison damage on a failed save, or half as much damage on a successful one. If the poison damage reduced the target to 0 hit points, the target is stable but poisoned for 1 hour, even after regaining hit points, and is paralyzed while poisoned in this way.

Web (Spider Form Only) (Recharge 5–6). Ranged Weapon Attack: +6 to hit, range 30/60 ft., one creature. Hit: The target is restrained by webbing. As an action, the restrained target can make a DC 13 Strength check, bursting the webbing on a success. The webbing can also be attacked and destroyed (AC 10; hp 5; vulnerability to fire damage; immunity to bludgeoning, poison, and psychic damage).