Yes: curses, ability damage, and ability drain causes loss of the ability to use the feat. You don't actually lose the feat, however - you just don't gain it's benefits until you meet the prerequisite again.
From the Players Handbook 3.5 page 87.
Prerequisites
A character cannot use a feat if he or she has lost the prerequisite. For example, if your characters strength drops below 13 because of a ray of enfeeblement spell, he or she can't use the Power Attack feat until the prerequisite is met again
When you lose a feat that is a prerequisite of another feat only the abilities of the prerequisite feat are lost, because you only have to have a prerequisite feat to qualify for another feat and losing the abilities doesn't change the fact that you have the prerequisite feat, you don't actually have to be able to use the prerequisite feat to qualify for the other feat...
...So if your dexterity was dropped below 13 you would lose access to Two-Weapon-Fighting
but you would not lose access to Oversized-Two-Weapon-Fighting.
Book of Exalted deeds gives the template of the risen martyr and the deathless creature:
Deathless is a new creature type, describing creatures that have
died but returned to a kind of spiritual life. They are similar in
many ways to both living creatures and undead. However,
while undead represent a mockery of life and a violation of the
natural order of life and death, the deathless merely stave off
the inevitability of death for a short time in order to accomplish
a righteous purpose. While undead draw their power
from the Negative Energy plane, the deathless are strongly tied
to the Positive Energy plane, the birthplace of all souls. In fact,
the deathless are little more than disincarnate souls, sometimes
wrapped in material flesh, often incorporeal and hardly
more substantial than a soul in its purest state.
Features
• 12-sided Hit Dice.
• Base attack bonus equal to 1/2 of total Hit Dice (as wizard).
• Good Will saves.
• Skill points equal to (4 + Int modifier, minimum 1) per
Hit Die, with quadruple skill points for the first Hit Die.
• No Constitution score.
• Darkvision out to 60 feet.
• Immunity to all mind-affecting effects (charms, compulsions,
phantasms, patterns, and morale effects).
• Immunity to poison, sleep effects, paralysis, stunning,
disease, and death effects.
• Not subject to critical hits, nonlethal damage, or
ability drain. Immune to damage to its physical
ability scores (Strength, Dexterity, and Constitution)
as well as fatigue and exhaustion effects.
Unlike undead, the deathless are subject to
energy drain. Like living creatures, deathless are
harmed by negative energy and healed by positive
energy.
• Immunity to any effect that requires a Fortitude
save, except for energy drain attacks, effects that
also work on objects, and harmless effects.
• Cannot use the run action.
• Uses Charisma modifier for Concentration
checks.
• Not at risk of death from massive damage, but
when reduced to 0 hp or less, it is immediately
destroyed.
• Not affected by raise dead and reincarnate spells or abilities.
Resurrection and true resurrection can affect the deathless if they
are willing. These spells turn deathless creatures back into the
living creatures they were before becoming deathless.
• Evil clerics can turn or destroy deathless creatures as good
clerics turn or destroy undead. Good clerics and paladins can
rebuke, command, or bolster deathless creatures as evil clerics
rebuke, command, or bolster undead.
• Deathless creatures gain the same benefits from
consecrate and hallow as undead do from desecrate
and unhallow, and they are hindered by desecrate
and unhallow as undead are by consecrate
and hallow. Hide from
undead and undeath to death
also work against deathless.
Detect undead and deathwatch also
reveal deathless, and allow the
caster to distinguish deathless
creatures from undead. Evil casters can be
stunned by overwhelming auras of deathless
creatures as good casters can be stunned
by overwhelming undead auras. Use the
“undead” line in the detect evil spell description
when deathless are in the area of a
detect good spell.
Deathless are healed by
disrupt undead and damaged by unholy
water as undead are by holy water.
Deathless are not affected by disrupting
weapons. Spells that have
greater than normal effect against
undead creatures—including chill
touch, magic stone, searing light,
sunbeam, sunburst, and wall of
fire—do not have these enhanced
effects against deathless
creatures. Deathless
take only 1d6 points of
damage per two caster
levels from searing light.
Spells such as command
undead, control
undead, create undead,
create greater undead,
and halt undead do not affect or create
deathless creatures.
• Proficient with its natural weapons and any weapons mentioned
in its entries.
• Proficient in whatever type of armor (light, medium, or
heavy) it is described as wearing, as well as all lighter types.
Deathless not listed as wearing armor are not proficient with
armor.
• Deathless do not breathe, eat, or sleep.
The Next Big one is the EIDOLON From ghostwalk
This class sub's levels and is perfect for a GM to transform a living party member or bring one back from the dead as a new class with a mission.
There is also the RISEN MARTYR from the Book of Exalted Deeds page 68:
This class is perfect for the GM to grant a single level in cause it's a prestige class and the first level is the hardest to achieve.
Best Answer
That I am aware of, there is no official template
If there was, it'd be in The Book of Vile Darkness, or else one of the Fiend Folios. I'm sad to say that I don't believe there's a template that mirrors Saint in terms of flavor or mechanics.
But...
I can propose some adjustments for you. Try doing this to the Saint template:
Requirements: Swap the requirement for a Good alignment with a requirement for an Evil one. Swap the prerequisite Exalted feats for Vile feats, and require a great act of evil in the name of a dark god (personal suggestion: a sacrifice of good-aligned sapient beings equal to double the character's hit dice).
Numerical Bonuses: Swap all instances of "perfection" bonuses with "profane" bonuses in the same areas. You can probably leave the energy resistance alone.
Spell-Like Abilities: Check and see which of the SLAs are distinctly "good" and replace them with evil ones. Some of them can probably stay, however, so don't go too crazy here.
Everything else, if I remember correctly, can remain the same.
Prestige Classes
There's a couple of prestige classes that might be useful to you if the villain in question is a fiend (an evil-aligned outsider) - namely, Fiend of Blasphemy and Fiend of Corruption, both of which can be found in the various Fiend Folios. These prestige classes give powers that help the fiend to corrupt, debase, and destroy souls in the names of greater patrons (or just Evil Itsownself) and could be very useful in your endeavor.