Demon's Sight.  Magical darkness doesn't impede the spider's darkvision.

Magic Resistance. The spider has advantage on saving throws against spells and other magical effects.

Sunlight Sensitivity. While in sunlight, the spider has disadvantage on attack rolls, as well as on Wisdom Perception checks that rely on sight.

Spider Climb. The spider 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 spider knows the exact location of any other creature in contact with the same web.

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

Multiattack. The spider makes four attacks: one with its fangs, one with it's web, and two with its legs.

Fangs. Melee Weapon Attack: +13 to hit, reach 5 ft., one creature. Hit: 29 (5d8 + 7) piercing damage, and the target must make a DC 21 Constitution saving throw, taking 27 (6d8) poison damage on a failed save and is poisoned and paralyzed for 1 hour, or half as much damage  on a successful save and is poisoned for 1 hour. If the poison damage reduces the target to 0 hit points, the creature is stable.

Legs. Melee Weapon Attack: +13 to hit, range 10 ft., one creature. Hit: 34 (6d8 + 7) piercing damage.

Web. Ranged Weapon Attack: +13 to hit, range 60/120 ft., one creature. Hit: 18 (4d8) acid damage.The target is restrained by webbing and takes 18 (4d8) acid damage at the end of each of it's turns while restrained. As an action, the restrained target can make a DC 21 Strength check, bursting the webbing on a success. The webbing can also be attacked and destroyed (AC 15; hp 25;  immunity to bludgeoning, poison, and psychic damage).

Multiple Legs. The Spider can take one reaction per turn with its legs, rather than only one per round (limit 8).