Ungoliant was the ancient spider from outside that drank the light from the Two Trees of Valinor and nearly slew Melkor / Morgoth. Since she fled Arda, she came to be worshipped by the Drow as Lolth. She is expected to return for Dagor Dagorath.

Outside sources:

Tolkien Gateway on Ungoliant

D&D fandom on Lolth