He means Lita the Witch, Mardero's mother.
Based on the name of the cliffs from which Lita was thrown, it seems quite possible that the demon who raped her was Hastur, the Lord of Nightmarea andArchangel of Mammon, God of Mind and Greed.
In the D&D campaign on which it was based, Lita was actually an Angel of Oghma. That gave her immunity to the effects of the fog with which Oghma had filled her vault to thwart Mammon's invasion. It drove any mortals or demons who entered it insane, making them forget who they were and what they were doing there. Oghma's libraries held an extremely important secret which Mammon wanted, but even if his forces could find it the mist would make them forget before they could return to tell him. Lita's rape was a way around this, as her children were powerful demons loyal to Mammon but whose angelic blood allowed them to pass through the mists safely. The equivalent of drowns were used by some mortals as these mindless thralls would remember their orders and fight through the mists, but they lacked the intelligence to actually find Oghma's secret. Many of Mammon's demons who did not remember who they were were reduced to mindless beasts, and many cults grew around worshiping specific crazed demons.
In FfH canon, Lita was a mortal who was raped because her Cambion children would not be as strictly bound by regulations of The Compact. True demons have no night to harm the righteous, but dispute supernatural power Cambions count as human and so can sin freely.