She almost appears to be a beautiful female Drow, with white hair cascading over glossy black skin, and four pairs of perfect almond eyes. From the waist down her body is that of a massive black spider, her carapace shining like polished marble.
At the time when the gods of the elves died, an Avatar of Llolth happened to be manifest on the material plane, deep in the Underdark delivering a message to a powerful Priest. By some chance, enough of Llolth’s power was present that she did not disappear when her parent goddess died, though her power was greatly diminished and her memory lost like so many others and she was trapped in the collapse of the temple. When she was freed almost a hundred years later, the nascent Drow civilization recognized her appearance from the ancient carvings, and though she didn’t remember who she was or why she looksed like the carvings it was easy for her to maneuver her way to a position of honorary Empress to unite the warring Drow city-states.
From there, she has employed craft and guile to gather more power to herself, until she was more than just a figurehead. Now she rules the Drow Empire with an iron claw, her secret police enforcing order among the populace as she plays the lesser politicians and religious leaders against each other. Her stated goal is to unite the Drow to conquer the rest of the Underdark, and amass an army to go out and conquer and enslave the surface world, but it is certain she has deeper plans and motives in play.
Llolth took the name of the fallen goddess as part of her political maneuvering, as it gave her a stronger claim to the crown, and as she has come to understand her nature she has decided that she is the rightful inheritor of the name anyway. She is not immortal, but she is close: She is immune to the effects of aging or disease, resistant to most poisons and regenerates damage very quickly, but she can die, and she knows it. She works very hard to encourage the rumor that she is the immortal goddess reborn though, as it discourages assassination attempts.