Lilianth of the Night Below is the demoness; her exact power level is unknown. According to her peer the Caged One, Lilianth is a Pit Fiend (the ultimate deadly demon) who occasionally masquerades as a Marilith (the penultimate deadly demon).
This engraving, titled The Death of Celowin Silvershield and the Wizard Abner, is believed to be an accurate representation of at least one form used by Lilianth.
Regardless of her precise designation, Lilianth is a formidable schemer. While staying at the palace of the Ninth Menegril in Limbo, Lilianth contrived to use the Grey Company to assassinate the Lich Patriki Van. She tinkered with the scrolls the party intended to trade to the lich, so that the scroll exploded when Van tried to read it. Lilianth's motive remains unknown, but the Assassins Guild of Limbo blamed the Grey Company for the unauthorized hit and has tried to settle the score.
To even accounts, Arnold Littleworth deceived the Mummy-Lich Lizard King into believing that Lilianth had defiled its temple. This defamation apparently offended Lilianth yet further, and she has evidently sworn to destroy the Grey Company and torment their souls in the afterlife.
The Caged One has implied that Lilianth occupies a high rank among the demons of Hell, but may be deposed, humiliated, or otherwise removed from office through shrewd tactis. Since Lilianth thus far has only made mischief, it seems ill-advised to invade the Netherworld.
Perhaps Ontussa can better explain the political shenanigans by which Lilianth rose to power at the Caged One's expense? This may prove useful in undermining her.
At one point, Obscura and Arnold Littleworth discussed seducing Lilianth with (from her perspective) taboo perversions such as true love and wholesome dating. This suggests an interesting, if extremely risky, carousal opportunity, but so far no one has dared it.