She uses her beauty as a bargaining chip, while Duncan goes along with the plan to explore their relationship and gather intel about the Dark Monarch.
As for her being one of the countless lovers of the Dark Monarch, at this point, who cares about such things anymore?
When Duncan asks, it will only be to compare who is fiercer, him or the Dark Monarch.
The answer is undoubtedly clear.
Since the Dark Monarch was imprisoned and lashed by a mortal witch for a period, upon returning to the Bottomless Abyss, he favored pampering males or genderless beings even more.
"It's the Zor Elves from City Menzoberranzan."
When Marita entered Zerata, she saw a group of tall elves hidden under cloaks. Though their dark skin was carefully concealed, it was still recognizable. This voluptuous Enchanting Mage seemed quite interested in Duncan and said softly, "They also have a caravan in Zerata."
"If you're interested."
"I can help you find a Zor Elf slave."
