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.