Here's my crack at it for 3.5 D&D. No 4E stuff around here buddy.
Dark Pyre Creature
Dark Pyre creatures dwell in regions or places ravaged by negative energy and fire of intense power. These charred skeletons burn with an unholy fi re and liquid flame pours from empty eye sockets like tears. The fiery nimbus of flames glow with an unnatural aura that unnerves even the brave.
Creating A Dark Pyre Creature
"Dark Pyre" is an inherited template that can be added to any corporeal aberration, animal, dragon, fey, giant, humanoid, magical beast, monstrous humanoid, ooze, plant, or vermin (referred to hereafter as the base creature).
A Dark Pyre creature uses all the base creature’s statistics and abilities except as noted here. Do not recalculate the creature’s Hit Dice.
Size and Type
Creatures with this template become augmented undead and acquire the fire subtype. Size is unchanged.
Special Attacks
A dark pyre creature retains all the special attacks of the base creature and also gains the following special attack.
Dark Pyre Corona (Su): Once every 5 rounds, as a standard action, a dark pyre creature can release blast of black fire, dealing (1/2 HD)d6 points of fire and (1/2 HD)d6 points of negative energy damage to each creature within 5 ft. (Reflex DC 10 + 1/2 dark pyre’s racial HD + Cha modifier negates). This heals (1/2 HD)d6 points of damage to any undead in the area, including itself.
Dark Pyre Strike (Su): Each of a dark pyre creature’s melee attacks deal +1d6 points of fire damage and +1d6 points of negative energy damage. The dark pyre skeleton’s natural weapons and any weapons it wields are treated as evil-aligned for overcoming damage reduction.
Firewalk (Su): A dark pyre skeleton can climb an object on fire as though it had a climb speed equal to its highest speed. Furthermore, it can fly at its highest speed with perfect maneuverability so long as it remains in contact with fire, and it can walk on flames at its highest speed, as though walking on air via the air walk spell.
Special Qualities
A dark pyre creature retains all the special qualities of the base creature and also gains the following.
* HD 5 or less, DR5/bludgeoning; HD 6 or more, DR10/bludgeoning and magic
* Natural Armor +3
* Immune to fire
* Improved Initiative feat
If the base creature already has one or more of these special qualities, use the better value.
The dark pyre creature’s natural weapons are treated as magic weapons for the purpose of overcoming damage reduction.
Dark Pyre Healing (Su): Dark pyre skeletons heal 1 point of damage for every 10 points of fire damage they would take from a given attack. Attacks that do less than 10 damage do not heal them.
Unnatural Aura (Su): Any animal within 30 feet of a dark-pyre skeleton must make a Will save (DC 10 + 1/2 dark pyre’s racial HD + Cha modifier) or become frightened. It only needs to flee beyond 30 ft. from the skeleton, at which point it is only shaken.
Water Vulnerability (Ex): A dark pyre skeleton takes damage from any sort of water as if it were acid. Holy water deals an additional 1d6 damage.
If the base creature’s Int is 3 or more, it also gains:
Spell-Like Abilities (Sp): 3/day – wall of fi re (DC 10 + 1/2 dark pyre’s racial HD + Cha modifier). At will – produce flame. Caster level is base creature’s HD.
Abilities
A dark pyre creature’s abilities improve as follows, Str +8, Dex +2, Con –, Int unchanged unless base creature’s Int is 2 or less then Int becomes 1, Wis –6, Cha unchanged
Base save bonuses are Fort +1/3 HD, Ref +1/3 HD, and Will +1/2 HD + 2.
Environment
Any fire infused location.
Challenge Rating
As base creature +1, but if the party’s APL is 4 or less the dark pyre template is a very difficult challenge.
Alignment
Alignment moves to Evil along the Good/Neutral/Evil axis
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Comments, suggestions and error-checking is always welcome. After writing this up, I feel the CR should be more the +1 since it gets a lot of powers with little weaknesses. Then again, at higher levels, these powers aren't as troublesome to overcome.
To whit, I wrote an LG mod with ability drain undead at APL 4 (4th level characters) and nearly got TPKs at all tables. Higher level parties didn't experience troubles. Just my humble opinion.