You want power

"I need power," you tell Libitina.

"You do have power, and strength, my son That's why you're the king."

"It's not enough, at least to defeat the succubus."

"Perhaps, but remember, everything has a price. Your armor, your sword, even your position. The question is, are you willing to pay it?"

"Tell me."

"I can help you achieve strength, but be forewarned that just as you become stronger in one aspect, you will become weak in another."

"Like my father?"

"His self doubt is of his own doing, unfortunately. Besides, the spells I weaved for him didn't have the same drawbacks. However, they didn't have the same potency and lasting effect."

So your father had to keep Libitina around to maintain strength, something he didn't, and that led to his downfall. The offer to you meant that it would be permanent, despite the implied drawbacks. "Why are you offering me this?"

Libitina let out a melodic laugh. "Think of this as me showing fondness to you as a mother would to a son."

You think for a while before nodding. "I accept."

Libitina claps a few time and three females appear. The first one is dressed in a castle maid's uniform. The second one is scantily clad, which hints at her less than desirable profession. Compared to the first two, the third one is dressed normally, like you would expect from someone with a good upbringing. "Choose to lay with any of them, and you will gain the strength you seek. That is the price."