Uraccen himself is in for being a part of the Forsworn- a group he joined after he was framed for the murder of a noble he was guarding and was forced to flee Markarth in the first place. He does seem to have bought in to the Forsworn ideals, though, claiming things were ‘better’ when the Reach was under Forsworn rule and looking forwards to the day that the city of Markarth is painted in Nord blood.
He can also tell you about the guards, who apparently only enter the mine once a week to haul out the dead, beat on those who instigated the trouble resulting in dead, take any ore that’s been mined, and bring in food- but if not enough ore has been mined, they don’t bring in any food at all. This jail being a harsh place, you’ll likely want to ask after the shiv and skooma he mentioned in his initial explanation- the shiv for yourself and the skooma to trade for anything you consider valuable enough.
Skooma, as you might expect, is really the only currency in the prison, and only shows up from outside. The shiv, thankfully, can be obtained from Grisvar, who apparently has an extra.
Uraccen can tell you about Madanach, but not much. If you ask after the King in Rags, Uraccen will realize that you’re the latest person to be thrown in here for life, and let you know that nobody gets to see Madanach. To do so, you’d have to get past Borkul the Beast, the imposing Orc in the room with you. According to Uraccen, Borkul is an especially big and violent Orsimer, who once ripped a man’s arm off so he’d have something to beat the guy to death with.
There’s a bit to be gained in here and Borkul is likely to be the last thing you deal with before facing Madanach himself, so pick up a pickaxe from the floor nearby and have yourself a search around the mine.
First off, the mine contains five separate silver veins, so you can collect yourself ten ore and possibly some gems as well for later use in smithing. There are also, of course, a rather ridiculous number of pickaxes, so you can gather a bunch of those if you like them or just need stuff to enchant for practice over and over again.