I don't know if I'm breaking any kind of copyright laws, but here goes:
DEATH KNIGHT (template)
A death knight is a horrific form of a lich created by a demon prince (it is thought Demogorgon) from a fallen paladin or favored blackguard. There are only twelve of these creatures known to exist.
A death knight appears as a skeletal form dressed in the same armor the character wore in life. The death knight’s eyes have decayed and pinpoints of crimson light burn in their place. Its clothes and armor usually show signs of wear and age.
A death knight speaks any languages it knew in life.
CREATING A DEATH KNIGHT
“Death Knight” is a template that can be added to any humanoid paladin (fallen) of at least 9th level (or blackguard of at least 3rd level) (referred to hereafter as the “character”). The character’s type changes to “undead.” It uses all the character’s statistics and abilities except as noted here.
Hit Dice: Increase to d12
Speed: Same as the character.
AC: The death knight has +2 natural armor or the character’s natural armor, whichever is better.
Special Attacks: A death knight retains all the character’s special attacks and also gains those listed below. Saves have a DC of 10 + 1/2 death knight’s HD + death knight’s Charisma modifier unless noted otherwise.
Fear Aura (Su): 5-foot radius, Will save or be affected as though by fear as cast by a sorcerer of the death knight’s level.
Spell-Like Abilities: At will—detect magic, see invisible, and wall of ice; 2/day—dispel magic; 1/day—fireball, any one power word spell and symbol (pain or fear effect only). All are as the spells cast by a 20th-level sorcerer (save DC 10 + death knight’s Cha modifier + spell level).
Summon Demons (Sp): Once per day a death knight can attempt to summon 2d8 dretches or 1d4 vrock with a 35% chance of success, or one hezrou, marilith or nalfeshnee with a 20% chance of success.
Special Qualities: A death knight retains all the character’s special qualities and gains those listed below, and also gains the undead type (see page 6, Monster Manual).
Darkvision (Ex): Range 60 feet.
Turn Resistance (Ex): A death knight has +4 turn resistance (see page 10, Monster Manual).
Spell Resistance (Ex): A death knight has SR 16 +1 per character level.
Summon Nightmare (Sp): Once per 10 years, a death knight may summon a nightmare (Nightmare, page 140 in the Monster Manual) to serve it as a paladin’s mount serves.
Saves: Same as the character
Abilities: A death knight gains a +4 to Strength and a +2 to Wisdom and Charisma, but being undead, has no Constitution score.
Skills: Death knights gain a +8 racial bonus to Intimidate, Sense Motive, and Spot checks. Otherwise same as the character.
Feats: Same as the character
Climate/Terrain: Any land and underground
Organization: Solitary
Challenge Rating: Same as the character +3
Treasure: None
Alignment: Any evil
Advancement: By character class
SAMPLE DEATH KNIGHT
This example uses a 9th-level paladin (fallen) as the character.
Death Knight
Medium-Size Undead
Hit Dice: 9d12 (58 hp)
Initiative: +5 (+1 Dex, +4 Improved Initiative)
Speed: 20 ft
AC: 22 (+1 Dex, +9 armor, +2 natural)
Attacks: +2 Longsword +11/+6 melee
Damage: +2 Longsword 1d8+7
Face/Reach: 5 ft by 5 ft/5 ft
Special Attacks: Spell-like abilities, fear aura, summon demons
Special Qualities: Undead, SR 25, turn resistance +4, summon nightmare, darkvision 60 ft
Saves: Fort +6, Ref +4, Will +6
Abilities: Str 20, Dex 13, Con –, Int 12, Wis 16, Cha 17
Skills: Diplomacy +12,Heal +12, Intimidate +11, Listen +8, Ride +12, Sense Motive +9, Spot +11
Feats: Cleave, Great Cleave, Improved Initiative, Power Attack
Climate/Terrain: Any land or underground
Organization: Solitary
Challenge Rating: 12
Treasure: Standard plus +1 full plate armor and +2 longsword
Alignment: Chaotic evil
Advancement: By character class
COMBAT
Fear Aura (Su): 5-foot radius, Will save (DC 17) or be affected as though by fear as cast by a sorcerer of the death knight’s level.
Spell-Like Abilities: At will—detect magic, see invisible, and wall of ice; 2/day—dispel magic; 1/day—fireball, any one power word spell and symbol (pain or fear effect only). All are as the spells cast by a 20th-level sorcerer (save DC 13 + spell level).
Summon Demons (Sp): Once per day a death knight can attempt to summon 2d8 dretches or 1d4 vrock with a 35% chance of success, or one hezrou, marilith or nalfeshnee with a 20% chance of success.
Summon Nightmare (Sp): Once per 10 years, a death knight may summon a nightmare (Nightmare, page 140 in the Monster Manual) to serve it as a paladin’s mount serves.
Undead: Immune to mind-influencing effects, poison, sleep, paralysis, stunning, and disease. Not subject to critical hits, subdual damage, ability damage, energy drain, or death from massive damage.
The Death Knight first appeared in the 1e FF (1981).
I think it's really quite powerful. Maybe too much so for a 3 level adjustment, but you wise men/women/sexless aliens be the judge on that.