Adult Fantasy (dark) about a wizard in a dungeon trying to get revenge

[Kardo]

Hi,
So I think I read it about 10 years ago.
It was divided to several characters, I think 3.
The mage\wizard was in a dungeon and I think it was under the palace of an emperor or something who was immortal.
Then there was a girl who was conscripted to a mage school. And I think they siphoned magic from other people to do more magic.
And then there was someone going somewhere but the mage wanted him to come to the dungeon.

[Kris]

Karod, is this book for teens or adults?

[Kardo]

Kris wrote: "Karod, is this book for teens or adults?"

for adults
it was pretty dark

[Kardo]

I've been thinking more on it.
I think he tried to get 3 people to him who he wanted to use to defeat the emperor.

I think he was there a long time. I think he used to be tortured (not sure, can't remember) but was then forgotten (or ignored) by the emperor (or king etc) and just sat in his cell plotting his revenge.

[Lobstergirl, au gratin]

Kardo is still looking and wrote:

"I started reading it in about 2008-2012 but for some reason stopped.
I think there were 3 storylines.
First is a mage kept in a dungeon/prison. He had been tortured and then forgotten. He uses his powers to call people to him who could help him get revenge against the immortal/eternal(?) who lives in the castle above the dungeon and imprisoned him.
A girl who is selected from a village/town for having magic. They take her to an academy where young mages are taught to suck magic from criminals (or slaves?) to use magic. I think they suck life force. Girl doesn't like it and has to escape because she might be used as a magic source if she disobeys.
Third was a man. Don't remember much. I think he lived in a fishing village. He somehow meets the girl and they start on a journey toward the mage."

[Claire]

Elminster: The Making of a Mage https://www.goodreads.com/book/show/5...

[Kardo]

Claire wrote: "Elminster: The Making of a Mage https://www.goodreads.com/book/show/5..."

No, not that