When I noticed in Faiths of Corruption that Mhar grants the Earth and Fire domains (previously only Brigh granted both) I had to make use of it for a concept I wanted to play on for a while.
The idea is that before Mhar himself gets free, his harbinger arrives on Golarion. A powerful leader of a cult from before the time of Thassilon, the Harbinger of Mhar was trapped accidentally during a ritual to free his master.
The Harbinger is a half fiend troll, with 20 levels of cleric, and the Earth and Fire domains, and the Die Hard feat.
He'd come into the campaign after the defeat of Karzoug, when the heros have left the Eye of Avarice. Some event has now freed the Harbinger - should not require the full event that would free Mhar, so he can be used even without that.
The Harbinger is much smarter and more cunning than other trolls, though he still prefers physical combat and eating the flesh of his victims. Now that he's back in the world, his ultimate goal is to again free Mhar... though if need be, he might start by rebuilding his cult again.
Since he is immune to fire and acid, there is no way to stop him from regenerating, and with the Die Hard feat he can't be knocked unconscious. The party will have to find some way to trap him again, or kill him without damage (starvation or suffocation for example).
I haven't finished statting him up, but I'd appreciate it if someone could check over what I have.
Haringer of Mhar (CR 25)
Half fiend troll cleric of Mhar 20
XP 1,638,400
CE Large outsider (native)
Init +2; Senses darkvision 60 ft., low-light vision, scent; Perception +14
Defense
AC 17, touch 11, flat-footed 15; (+2 Dex, +6 natural, –1 size)
hp 325 (26d8+208); regeneration 5
Fort +25, Ref +10, Will +21
Immune poison, acid, fire
Electricity, cold resistance 10
DR 10/magic
SR 35
Offense
Speed 30 ft., Fly 60 ft. (Good)
Melee bite +26 (1d8+8), 2 claws +26 (1d6+8)
Space 10 ft.; Reach 10 ft.
Special Attacks rend (2 claws, 1d6+11), Smite Good (+2 attack/AC, +26 damage, 1/day), Acid Dart (1d6+10, 8/day), Fire Bolt (1d6+10, 8/day)
Spell Like Abilities (CL 26):
3/day - Darkness, Poison, Unholy Aura
1/day - Desecrate, Unholy Blight, Contagion, Blasphemy, Unhallow, Horrid Wilting, Summon Monster IX (Fiends only), Destruction
Cleric Spells (CL 20)
0th - 4
1st - 6+1
2nd - 5+1
3rd - 5+1
4th - 5+1
5th - 5+1
6th - 4+1
7th - 4+1
8th - 4+1
9th - 4+1
Channel Negative Energy (10d6, DC 22)
Domains Earth, Fire
Statistics
Str 26, Dex 14, Con 26, Int 12, Wis 20, Cha 14
Base Atk +19; CMB +27; CMD 40
Feats Intimidating Prowess, Iron Will, Skill Focus (Perception), Medium Armor, Shields, Simple Weapons, Heavy Pick, Die Hard, 9 more
SQ Aura (CE)
Skills Intimidate +13, Perception +14, 102 unused skill points
Languages Abyssal
Ecology
Environment Mhar Massif
Organization solitary
Treasure NPC Gear CR 25 (Ring of sustenance?)
Dotted for awesome
Mean. Mean mean mean.
If I had to face this jerk I'd want to be a Wizard. Greater Spell Penetration, Greater Spell Focus Necromancy and Heightened Magic Jar/ Heightened Trap The Soul gives me a pretty good chance of being able to just be this guy.
Hmm, just realized I didn't give him the Endurance feat, which is required for Diehard. :D
Doomed Hero, I'm not 100% sure that would work... his immunity to fire and acid come from his cleric levels. If you inhabit his body, does it lose those traits?
Edit: The more I think about it, the more I think this should be the big bad of a sequel AP. The bulk of the AP follows him rebuilding his cult and finding the magic doohickey needed to finish his ritual and free Mhar, with one adventure given to finding a way to actually retrap him. (Suffocation and starvation actually don't work after all, since regeneration flatly says you don't die.)