The Unholy Warrior
From The Unholy Warrior's Handbook by Green Ronin Publishing:

Unholy Warrior Overview

Religion: If a power is evil, offering satiation for mortal desires and rewards for service, in all likelihood there is an unholy warrior in service to it. Unlike other classes, unholy warriors uniformly uphold the particular edicts associated with whatever foul religion they follow. It matters not if the power they uphold originated from any of the fiends of the lower planes, or even a fallen god.

Abilities: Charisma is the defining statistic determining one's capacity for self-realization, and is the key to many of the unholy warrior's skills of deception and treachery. Many of the domain powers granted to holy warriors determine their potency on this attribute; for unholy warriors, Charisma is the determining factor for all things related to casting spells, including level availability, DCs, and bonus spells. Next to Charisma, the unholy warrior should focus on Strength, as this attribute is vital to combat. Finally, Constitution increases hit points, aids in Concentration checks, and is the determining factor in resisting the many poisons and diseases to which this character is exposed.

Alignment: Any evil.

Hit die: d10

All of the following are class features of the Unholy Warrior core class:

Weapon and Armor: Unholy Warriors are proficient with all simple and martial weapons. They are proficient with all armor and shields.

Domain-Based Special Abilities: Unholy warriors gain domains granted by whichever dark power they worship. Unlike clerical domains, these abilities are not enhanced spell lists and granted powers. Instead, they reflect the martial nature of the unholy warrior and mirror many of the domains granted to their diametric opposite, the holy warrior.

At first level, the unholy warrior may choose two domains from those available to him. This grants four abilities, two from each domain. At 2nd level, he gains an additional ability from each domain. Thus, by 2nd level, the unholy warrior has six domain abilities.

Rebuke Undead: At 3rd level, the unholy warrior gains the ability to rebuke and command undead, as a cleric of two levels lower would. This ability can be used a number of times per day equal to 3 + Charisma modifier. There may be additional effects available in the campaign setting, allowing the channeling of negative energy in unusual but useful ways such as boosting spells, preparing sacrifices, and other twisted actions. Consult such sources if they are used in your particular gaming group. Unholy warriors may also take the Extra Rebuking feat to rebuke more times per day. This feat works in the same manner as Extra Turning, except it adds four additional rebuking attempts in lieu of the turning attempts. Unholy warriors, if so inclined, may take this feat multiple times, each time gaining an additional four attempts per day.

Spells: Beginning at 4th level, the unholy warrior gains the ability to cast a small number of divine spells. To cast a spell, an unholy warrior must have a Charisma score of at least 10 + the spell's level, so an unholy warrior with a Charisma of 10 or lower cannot cast these spells. Unholy warriors derive bonus spells from their Charisma scores; saving throws against these spells have a DC of 10 + spell level + the unholy warrior's Charisma modifier. If the unholy warrior receives 0 spells for a particular level, such as 0 1st-level spells at 4th level, she gets only bonus spells. Thus, without a bonus spell for that same level, she cannot cast a spell of that level. The unholy warrior's spell list is determined by the power he worships. Like a cleric, the unholy warrior may prepare any spell on the list of the appropriate level from which she normally casts, and casts those prepared spells in the same way as a cleric. Unholy warriors do not gain the ability to cast spontaneous injury spells.

Through 3rd level, an unholy warrior has no caster level. Starting at 4th level, an unholy warrior's caster level is one-half his class level.

Gifts of Darkness: Starting at 5th level, unholy warriors gain rewards for their services to the powers of evil. At each level where an unholy warrior gains a Gift of Darkness, she receives a supernatural or spell-like ability as defined by his particular faith.

Dark Ally: At 6th level, the dark forces lend a foul ally to serve the unholy warrior on his quest to spread misfortune. The unholy warrior may choose to call this companion at any point during or after this level. The unholy warrior's master determines the exact nature of the companion insofar as form, alignment, and other abilities. As a default reward, the unholy warrior may summon a fiendish warhorse. All dark allies receive benefits based upon the level of the unholy warrior as described in the below table. Unlike holy warriors and paladins, the dark ally is a tool. Despite the master/slave relationship, there is no empathic link between the steed and the rider. The dark ally is hereafter a magical beast regardless of its base creature type.

Forbidden Seven: No universal formal code of conduct exists for unholy warriors. What boundaries do exist take the form of the seven virtues, referred to as the Forbidden Seven. If the unholy warrior deviates from evil and performs any of the forbidden acts knowingly and with good intention, his patron will strip him of all his abilities. Only seven possible actions, performed in a manner disturbing to the unholy warrior's patron, exist: faith, hope, love, prudence, justice, fortitude, and temperance. A character that performs one of the forbidden seven acts may retain his abilities if subject to atonement, provided an evil cleric casts the spell within a time frame of days equal to unholy warrior class levels. So, a 3rd level unholy warrior who commits an act of love has three days to atone before he is stripped of all of his abilities. Consult ex-unholy warriors, below, for details on this new status.

Granted, it may be necessary to commit any one of the forbidden seven, especially with treachery in the heart. Despite the risks, the reward for betraying someone with whom you have built trust and rapport is much sweeter when the knife blade is unsuspected. If used for evil, any of the forbidden seven are viable options to corrupt and destroy good, and thus are not instances where the unholy warrior could be stripped of his powers. Unholy warriors who knowingly perform a good act, such as exhibiting a virtue associated with one of the forbidden seven, abandon the path of the unholy warrior. Unless they atone, they become ex-unholy warriors. Characters voluntarily changing their alignments are also stripped of their powers. Ex-unholy warriors lose all class abilities and access to spells, and may never gain further levels in the unholy warrior class.