The Lord of the Iron Citadel.
And so the prestige class would be Disciple of Dispater. I know the usual wariness regarding stacking crit range amplifications (specially with a Falcata, which would end at -quick calculations- 15-20 without Improved Critical, 13-20 with it... too much by my account) and as such I wouldn't be against modifying it to not stack with Improved Critical or whatever changes you deign necessary.
To be honest, the PrC's main appeal for me are the SLAs (Summon Erynie's sounds fun as hell, as well as Rusting Grasp)
So, just so you can convert it, my idea is to go straight fighter (perhaps a level of two of ranger/rogue or such if RP appropiate) and take Disciple of Dispater when the RP allows it, leaving the remote posibility of taking Ur-Priest at mid to high levels if you later allow it and, once again, the RP allows it organically.
TL;DR: Disciple of Dispater and Dispater it is, as Ur-Priest is something I'd take at too high a level to think about it now.