I have always liked the idea of a blackguard, the mirror to the holy paladin. However, in 3.5 they made it a prestige class when I feel it should have been a full class, so here is how I feel the blackguard would look in Pathfinder.

Few evil creatures would call themselves evil, and fewer still would champion it. Those select few are the blackguards. Blackguards champion either and evil god, tyranny, or evil itself believing it is needed to balance out good. A blackguard is a blight upon the land, bringing fear, misery, and destruction wherever they go. Many powerful blackguards seek positions of power, either over a regiment of like minded soldiers, or over a town or city, using it for his own dark desires. Unlike most evil creatures, a blackguard is fearless and will die for his cause.

Role: Blackguards rarely travel alone. They lead like minded companions into battle, using them to support his cause. They prefer to leap into the front of battle and using their unholy abilities to kill, maim, and poison their enemies.

Alignment: Lawful Evil

Hit Die: d10.

Class Skills

The sorcerer's class skills are Bluff (Cha), Craft (Int), Diplomacy (Cha), Handle Animal (Cha), Heal (Wis), Intimidate (Cha), Knowledge (nobility) (Int), Knowledge (religion) (Int), Profession (Wis), Ride (Dex), Sense Motive (Wis), and Spellcraft (Int).

Skill Ranks per Level: 2 + Int modifier.

Class Features

All of the following are class features of the blackguard.

Weapon and Armor Proficiency: Blackguards are proficient with all simple and martial weapons, with all types of armor (heavy, medium, and light), and with shields (except tower shields).

Aura of Evil (Ex)

The power of a blackguard's aura of evil (see the detect evil spell) is equal to his blackguard level.

Detect Good (Sp)

At will, a blackguard can use detect good, as the spell. A blackguard can, as a move action, concentrate on a single item or individual within 60 feet and determine if it is good, learning the strength of its aura as if having studied it for 3 rounds. While focusing on one individual or object, the blackguard does not detect good in any other object or individual within range.

Smite Good (Su)

Once per day, a blackguard can call out to the powers of good to aid his in his struggle against good. As a swift action, the blackguard chooses one target within sight to smite. If this target is good, the blackguard adds her Cha bonus (if any) to his attack rolls and adds her blackguard level to all damage rolls made against the target of his smite. If the target of smite good is an outsider with the good subtype, an good-aligned dragon, or an undead creature, the bonus to damage increases to 2 points of damage per level the blackguard possesses. Regardless of the target, smite good attacks automatically bypass any DR the creature might possess.

In addition, while smite good is in effect, the blackguard gains a deflection bonus equal to his Charisma modifier (if any) to his AC against attacks made by the target of the smite. If the blackguard targets a creature that is not good, the smite is wasted with no effect.

The smite good effect remains until the target of the smite is dead or the next time the blackguard rests and regains his uses of this ability. At 4th level, and at every three levels thereafter, the blackguard may smite good one additional time per day, as indicated on Table: Blackguard, to a maximum of seven times per day at 19th level.

Divine Grace (Su)

At 2nd level, a blackguard gains a bonus equal to his Charisma bonus (if any) on all Saving Throws.

Painful Touch (Su)

Beginning at 2nd level, a blackguard can inflict wounds to living creatures by touch. This requires a successful melee touch attack and doesn't provoke an attack of opportunity. Creatures do not receive a saving throw against this damage. Each day he can use this ability a number of times equal to 1/2 his blackguard level plus his Charisma modifier. With one use of this ability, a blackguard can deal 1d6 hit points of damage for every two blackguard levels he possesses. Despite the name of this ability, a blackguard only needs one free hand to use this ability.

Alternatively, a blackguard can use this harming power to deal damage to undead creatures, healing 1d6 points of damage for every two levels the blackguard possesses. At this level a blackguard heals from negative energy as an undead and is damaged by positive energy. Using this ability this way is a standard action, unless the blackguard targets herself, in which case it is a swift action.

Aura of Fear (Su)

At 3rd level, a blackguard is immune to fear (magical or otherwise). Each enemy within 10 feet of his must make a Will save equal to 10 + half the blackguard's level + the blackguard's Charisma modifier or become shaken for one round. This increases to 1d4 rounds at 10th level. This ability functions only while the blackguard is conscious, not if he is unconscious or dead.

Divine Health (Ex)

At 3rd level, a blackguard is immune to all diseases, including supernatural and magical diseases.

Cruelty (Su)

At 3rd level, and every three levels thereafter, a blackguard can select one cruelty. Each cruelty adds an effect to the blackguard's painful touch ability. Whenever the blackguard uses painful touch to deal damage to one target, the target must make a Fortitude save for each cruelty (10 + half the blackguard's level + the blackguard's Charisma modifier) or receive one additional effect from one of cruelties possessed by the blackguard. Duration is 1 round per blackguard level. At 8th level and every four levels thereafter, the black guard can inflict one additional cruelty upon their target. Once a condition or spell effect is chosen, it can't be changed.

At 3rd level, the blackguard can select from the following initial cruelties.

Fatigued: The target is Fatigued.

Shaken: The target is Shaken.

Sickened: The target is Sickened.

At 6th level, a blackguard adds the following cruelties to the list of those that can be selected.

Dazed: The target is Dazed.

Diseased: The blackguard's painful touch ability also acts as contagion, using the blackguard's level as the caster level.

Staggered: The target is Staggered.

At 9th level, a blackguard adds the following cruelties to the list of those that can be selected.

Cursed: The blackguard's painful touch ability also acts as bestow curse, using the blackguard's level as the caster level.

Exhausted: The target is Exhausted. The blackguard must have the fatigue cruelty before selecting this cruelty.

Frightened: The target is Frightened. The blackguard must have the Shaken cruelty before selecting this cruelty.

Nauseated: The target is Nauseated. The blackguard must have the Sickened cruelty before selecting this cruelty.

Poisoned: The blackguard's painful touch ability also acts as poison, using the blackguard's level as the caster level.

At 12th level, a blackguard adds the following cruelties to the list of those that can be selected.

Blinded: The target is Blinded.

Deafened: The target is Deafened.

Paralyzed: The target is Paralyzed.

Stunned: The target is Stunned.

Channel Negative Energy (Su)

When a blackguard reaches 4th level, he gains the supernatural ability to channel negative energy like a cleric. Using this ability consumes two uses of his lay on hands ability. A blackguard uses his level as his effective cleric level when channeling negative energy. This is a Charisma-based ability.

Spells

Beginning at 4th level, a blackguard gains the ability to cast a small number of divine spells which are drawn from the blackguard spell list. A blackguard must choose and prepare his spells in advance.

To prepare or cast a spell, a blackguard must have a Charisma score equal to at least 10 + the spell level. The Difficulty Class for a saving throw against a blackguard's spell is 10 + the spell level + the blackguard's Charisma modifier.

Like other spellcasters, a blackguard can cast only a certain number of spells of each spell level per day. His base daily spell allotment is given on Table: Blackguard. In addition, he receives bonus spells per day if he has a high Charisma score (see Table: Ability Modifiers and Bonus Spells). When Table: Blackguard indicates that the blackguard gets 0 spells per day of a given spell level, he gains only the bonus spells he would be entitled to based on his Charisma score for that spell level.

A blackguard must spend 1 hour each day in quiet prayer and meditation to regain his daily allotment of spells. A blackguard may prepare and cast any spell on the blackguard spell list, provided that he can cast spells of that level, but he must choose which spells to prepare during his daily meditation.

Through 3rd level, a blackguard has no caster level. At 4th level and higher, his caster level is equal to his blackguard level – 3.

Divine Bond (Sp)

Upon reaching 5th level, a blackguard forms a divine bond with his god. This bond can take one of two forms. Once the form is chosen, it cannot be changed.

The first type of bond allows the blackguard to enhance his weapon as a standard action by calling upon the energy of an infernal soul for 1 minute per blackguard level. When called, the soul causes the weapon to shed light as a torch. At 5th level, this soul grants the weapon a +1 enhancement bonus. For every three levels beyond 5th, the weapon gains another +1 enhancement bonus, to a maximum of +6 at 20th level. These bonuses can be added to the weapon, stacking with existing weapon bonuses to a maximum of +5, or they can be used to add any of the following weapon properties: axiomatic, brilliant energy, defending, disruption, flaming, flaming burst, keen, speed, unholy,and wounding. Adding these properties consumes an amount of bonus equal to the property's cost (see Table: Melee Weapon Special Abilities). These bonuses are added to any properties the weapon already has, but duplicate abilities do not stack. If the weapon is not magical, at least a +1 enhancement bonus must be added before any other properties can be added. The bonus and properties granted by the soul are determined when the soul is called and cannot be changed until the soul is called again. The infernal soul imparts no bonuses if the weapon is held by anyone other than the blackguard but resumes giving bonuses if returned to the blackguard. These bonuses apply to only one end of a double weapon. A blackguard can use this ability once per day at 5th level, and one additional time per day for every four levels beyond 5th, to a total of four times per day at 17th level.

If a weapon bonded with a infernal soul is destroyed, the blackguard loses the use of this ability for 30 days, or until he gains a level, whichever comes first. During this 30-day period, the blackguard takes a –1 penalty on attack and weapon damage rolls.

The second type of bond allows a blackguard to gain the service of an unusually intelligent, strong, and loyal steed to serve him in his crusade against good. This mount is usually a heavy horse (for a Medium blackguard) or a pony (for a Small blackguard), although more exotic mounts, such as a boar, camel, or dog are also suitable. This mount functions as a druid's animal companion, using the blackguard's level as his effective druid level. Bonded mounts have an Intelligence of at least 6.

Once per day, as a full-round action, a blackguard may magically call his mount to his side. This ability is the equivalent of a spell of a level equal to one-third the blackguard's level. The mount immediately appears adjacent to the blackguard. A blackguard can use this ability once per day at 5th level, and one additional time per day for every 4 levels thereafter, for a total of four times per day at 17th level.

At 11th level, the mount gains the fiendish creature advanced simple template and becomes a magical beast for the purposes of determining which spells affect it.

At 15th level, a blackguard's mount gains spell resistance equal to the blackguard's level + 11.

Should the blackguard's mount die, the blackguard may not summon another mount for 30 days or until he gains a blackguard level, whichever comes first. During this 30-day period, the blackguard takes a –1 penalty on attack and weapon damage rolls.

Aura of Undeath (Su)

At 8th level,  He also gains the ability to Control Undead as per the feat. Each undead ally or under his control within 10 feet of him gains a +2 bonus on saving throws against channeling.

This ability functions only while the blackguard is conscious, not if he is unconscious or dead.

Aura of Vengeance (Su)

At 11th level, a blackguard can expend two uses of his smite good ability to grant the ability to smite good to all allies within 10 feet, using his bonuses. Allies must use this smite good ability by the start of the blackguard's next turn and the bonuses last for 1 minute. Using this ability is a free action. Good creatures gain no benefit from this ability.

Aura of Corruption (Su)

At 14th level, a blackguard's weapons are treated as evil-aligned for the purposes of overcoming Damage Reduction. Any attack made against an enemy within 10 feet of him is treated as evil-aligned for the purposes of overcoming Damage Reduction.

This ability functions only while the blackguard is conscious, not if he is unconscious or dead.

Aura of Wickedness (Su)

At 17th level, a blackguard gains DR 5/good and SR 11+blackguard level. Each enemy within 10 feet of him takes -2 to their Armor Class from attacks from the blackguard.

This ability functions only while the blackguard is conscious, not if he is unconscious or dead.

Unholy Champion (Su)

At 20th level, a blackguard becomes a conduit for the power of his god. His DR increases to 10/good. Whenever he uses smite good and successfully strikes an good outsider, the outsider is also subject to a banishment, using his blackguard level as the caster level (his weapon and holy symbol automatically count as objects that the subject hates). After the banishment effect and the damage from the attack is resolved, the smite immediately ends. In addition, whenever he channels negative energy or uses painful touch to damage a creature, he deals the maximum possible amount.

Code of Conduct

A blackguard must be of lawful evil alignment and loses all class features except proficiencies if he ever willingly commits an good act.

Additionally, a blackguard's code requires that he respect legitimate authority, act with honor (not lying, not cheating, not using poison, and so forth), help those in need (provided they do not use the help for good or chaotic ends), and punish those who harm or threaten innocents.

Associates: While he may adventure with evil or neutral allies, a blackguard avoids working with good characters or with anyone who consistently offends his moral code. Under exceptional circumstances, a blackguard can ally with evil associates, but only to defeat what he believes to be a greater good. A blackguard should seek an atonement  spell periodically during such an unusual alliance, and should end the alliance immediately should he feel it is doing more harm than good. A blackguard may accept only henchmen, followers, or cohorts who are lawful evil.

Ex-Blackguards

A blackguard who ceases to be lawful evil, who willfully commits an good act, or who violates the code of conduct loses all blackguard spells and class features (including the service of the blackguard's mount, but not weapon, armor, and shield proficiencies). He may not progress any further in levels as a blackguard. He regains his abilities and advancement potential if he atones for his violations (see atonement), as appropriate.