Drider

Driders are drow who, by the power of Lolth, the Spider Queen, have been twisted into hybrids of drow and giant spiders.

Description
Driders appear like drow from the head to the waist, but the waist is connected to the thorax — and then abdomen and legs — of a giant spider. They typically carry a longbow with poisoned arrows and a longsword.

History
Leading up to the Calamity, Lolth had seduced the leaders of the increasingly decadent, cruel drow society, so that when the war began, the drow allied with her against Corellon and his elf followers. However, before the drow soldiers even saw battle, Kord impaled Lolth against a cliffside with one heave of his thunderspear, and Lolth was banished to the Abyss. The now-leaderless drow were driven from the surface world, and Lolth's silvery blood pooled in the dark caverns beneath the surface of the world. In the years that followed, her most devoted drow servants discovered the blood and drank of it deeply, granting them power and gradually twisting themselves into driders. The driders grew in number, but the blood began to run out, tormenting the driders and causing them to wander the Underdark seeking hidden pools of the blood.

Following the Divergence, Lolth's children could feel the lessening of their banished god's influence. The drow of Xhorhas mostly abandoned their centuries-long worship of Lolth, turning instead to the Luxon, whose power is inscrutable to the Spider Queen. The dark elves of Tal'Dorei were not so fortunate. Their underground empire-building exposed them to new dangers: Tharizdun began whispering to the drow nobility, and their servitors found themselves surrounded by shapechanging and mind-manipulating aberrations, causing mass paranoia. By 812 PD, the underground drow of Tal'Dorei were on the verge of collapse, and some drow nobility gave themselves over utterly to Lolth, willingly seeking out the blood of the Spider Queen to transform themselves into driders.

Traits and abilities
The following are features of driders according to the Monster Manual, 5th Ed., and may not be canon to Critical Role:

Driders each carry a longsword and a longbow. They can make three attacks with one action, using either weapon, and they can replace one of these attacks with a Bite attack. The longbow and bite both deal additional poison damage.
 * Fey Ancestry - Advantage on saving throws against being charmed, and magic can't put the drider to sleep.
 * Innate Spellcasting Using Wisdom (save DC 13), requiring no material components:
 * At will: dancing lights
 * 1/day each: darkness, faerie fire
 * Driders that were once spellcasters may have an expanded spell list.
 * Spider Climb - No ability check needed to climb difficult surfaces.
 * Sunlight Sensitivity While in sunlight, has disadvantage on attack rolls and on Perception checks that rely on sight.
 * Web Walker - Ignores movement restrictions caused by webbing.

Trivia
As of 835 PD, there were no mention of drow in the Lolthite Children of Malice transforming themselves into driders; that faction of spies, assassins, and raiders seems more focused on infiltration.