Fey Ancestry. The drider has advantage on saving throws against being charmed, and magic can't put the drider to sleep.

Innate Spellcasting. The drider's innate spellcasting ability is Wisdom (spell save DC 13). The drider can innately cast the following spells, requiring no material components:

At will: dancing lights, chill touch

1/day each: darkness, bestow curse

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

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

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

Undead Fortitude. If damage reduces the Undead Drider to 0 Hit Points, it must make a Constitution saving throw with a DC of 5+the damage taken, unless the damage is radiant or from a critical hit. On a success, the Undead Drider drops to 1 hit point instead.

Multiattack. The drider makes three attacks with its bonesword. It can replace one of those attacks with a bite attack.

Bite. Melee Weapon Attack: +6 to hit, reach 5 ft., one creature. Hit: 2 (1d4) piercing damage plus 9 (2d8) poison damage.

Bonesword. Melee Weapon Attack: +6 to hit, reach 5 ft., one target. Hit: 7 (1d8 + 3) slashing damage, or 8 (1d10 + 3) slashing damage if used with two hands.