Currently playing at around level 4, characters come to scorched, desecrated earth where the most recent incarnation of evil is taking hold (in the form of the BBEG #2 guy training necromancers in the fine art of "zombies tied to worgs, trained to not let the zombies fall off").
Characters have dispatched the previous 2 necromancer encampments with relative ease between a combination of Turn Undead, Grease, a Mace of Disruption, a summoner, and a katana-wielding Knight. They thought this would be another standard mission when they came to the goblin graveyard. Until they were hit with a Fireball...
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Conduit of the Lower Planes - Medium Fey Evil Outsider
Feats:
1 - Product of Infernal Dalliance (2 claws and a bite)
3 - Spines of Fury (2 dagger-like spines/round (natural attacks))
4 - Poison Sacs (DC14 Drow poison, Fort save or unconscious)
Misc abilities:
Immune to Compulsion Effects, Fear, and Fire
Vulnerable to Cold
Cold Resistance 5
Any 10HD creature or less slain becomes a zombie
Enhanced Spheres:
3x/day - Fireball (level 1 spell, so it has a miserable save)
3x/day - Command Undead
2x/day - Searing Ray
2x/day - Desecrate
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All in all just a very surprising encounter. The mechanics of getting a zombie to ride were fairly irrelevant because the cleric destroyed half of them the first round they were in range (the other half in the following two). Wall of Smoke (summoner) and Spike Stones (Spherelock) stopped the charge in it's tracks. I always dread for my campaigns to get caught in monotony, and the fireball took the party for a large surprise. He even got another off in between fending off summoned hippogriffs before the party could scatter (which animated one of the worgs).
I was just extremely pleased with the array of off-the-wall abilities to surprise characters with. A spellcaster that can claw, claw, bite, poison, throw spines (he didn't get to do this

Shame he didn't get away on the worg though (runs 200ft/round). Hippogriffs are damned fast fliers. Who the hell wrote in that they could CHARGE at 200 feet?