From Compendium of Worldly Lore
Ur-priests are to Clerics as Blackguards are to the noble and divine Paladin. These are essentially anti clerics that rebuke undead instead of turning them away. They steal the power from under the noses of the gods instead of having it freely gifted to them for obedience. No the Ur-priest is a more rebellious path than the Cleric. They revile the gods so thoroughly they gain resistance to divine magic in its entirety only being affected by it so should they wish to be. Their thievery does not end there as they eventually can steal away even inherent spell-like abilities.
Role: Spell Utility
Characteristics: Anti-priests stealing magic from the gods and gaining resistance to their spells
Source: Complete Divine
Game Rule Information
The Ur-priest has the following game statistics.
Abilities: Wisdom for your casting stat, you need Wisdom 19 to cast 9th level spells, Constitution for Health as your armor may or may not be great depending on previous classes
Alignment: Any Evil
Hit Die: d8
Starting Gold: N/A
Class Skills
The Ur-priest's class skills are Bluff (Cha), Concentration (Con), Craft (Int), Knowledge (arcana) (Int), Knowledge (religion) (Int), Knowledge (the planes) (Int), Profession (Wis), Scry (Int), Search (Int), Spellcraft (Int) and Spot (Wis)
Skill Points at 1st Level: (2 + Int modifier) × 4
Skill Points at Each Additional Level: 2 + Int modifier
Class Features
All of the following are class features of the Ur-priest.
Weapon and Armor Proficiency: A Ur-priest is proficient with all simple weapons, no armor
Spells per Day: An ur-priest gains the ability to cast a number of divine spells. To cast a spell, an ur-priest must have a Wisdom score of at least 10 + the spell's level, so an ur-priest with a Wisdom of 10 or lower cannot cast these spells. Ur-priest bonus spells are based on Wisdom, and saving throws against these spells have a DC of 10 + spell level. When the table indicates that the ur-priest gets 0 spells per day of a given spell level, such as 0 2nd-level spells at 2nd level, the ur-priest gains only the bonus spells he would be entitled to based on his Wisdom score for that spell level.
The ur-priest spell list is identical to the cleric spell list. An ur-priest has access to any spell on the list and prepares those spells as a cleric, except that he does not pray for spells, he just takes them. An ur-priest casts spells as a cleric does, except that unlike a cleric, he does not have the ability to spontaneously cast cure or inflict spells, nor does he have domain spells or associated domain granted powers. He does not have restrictions on spells with alignments. To determine the caster level of an ur-priest, add the character's ur-priest levels to one-half of his levels in other spellcasting classes. (Any levels gained in the cleric class by an ex-cleric don't count.)
Rebuke Undead: Like an evil cleric, an ur-priest of 2nd level or higher can rebuke undead. He uses his ur-priest level as the cleric level for determining success and damage.
Divine Spell Resistance Su: At 4th level, an ur-priest gains spell resistance 15, but only against divine spells and the spell-like abilities of outsiders. At 8th level, the ur-priest's spell resistance increases to 20.
Siphon Spell Power Ex: Because they steal whatever power they can, ur-priests learn to manipulate their energy in ways that confound other casters. An ur-priest of 6th level or higher can temporarily sacrifice two (or more) lower-level spell slots and use those slots to prepare a higher-level spell. The higher-level spell must be of a level the ur-priest can cast. Only one exchange of this sort can be made each day. The levels of the lower-level slots are totaled, then reduced to three-quarters (round down) to determine the level of the extra higher-level spell slot. For example, an ur-priest who sacrifices a 3rd-level spell and a 5th-level spell can use that spell slot to prepare an additional 6th-level spell (3 + 5 = 8, and 8 × 3/4 = 6).
Steal Spell-like Ability Su: The most powerful ur-priests can use the same techniques that siphon off the gods' spell power to steal the spell-like abilities of another creature. Once each day, when a creature with spell-like abilities is within 50 feet of a 10th-level ur-priest, the ur-priest can choose one of the spell-like abilities of the creature to steal for himself. The ur-priest can use the spell-like ability as often as the creature can, or three times per day, whichever is less. The ur-priest uses the ability as the creature does with regard to caster level and save DCs. This ability only lasts 24 hours. The creature with the spell-like ability does not lose the ability when the urpriest steals it. If an ur-priest tries to steal a spell-like ability that the creature doesn't have, or tries to steal an ability that is supernatural rather than spell-like, the attempt automatically fails.
For example, if an ur-priest is near a noble salamander, he can steal Fireball and use it three times that day or steal Dispel Magic and use it once that day. If he is near a pit fiend, he could steal Teleport Without Error (self plus 50 pounds of objects only) and use it three times per day, because the pit fiend can do it at will. He could even steal the pit fiend's Wish ability, but because a pit fiend can only use wish once per year, the ur-priest would be similarly limited. He could not steal that power again from any creature for one year.