Just some rambling ideas.
Wrath ties to a whole host of dark emotions like anger, hate, betrayal, guilt, etc... Fun emotions to roleplay with.
Here's a few ideas that I had. The plane (dimension?) of wrath is all canyons interconnected with each other. This keeps it in a outdoor setting but still has the twists and turns and openings of a dungeon setting. Now you can have bears running around in a more natural setting.
Oh and these bears aren't just any ole black bear. No, they have fiery red-orange-brown hair and stand 15-20 feet on their hind quarters. Big snaggle toothed Rampage bears. Give them cool stuff like rending capabilities. Give them the Cleave feet on their swiping claws. Give them rampage...a boosted natural ability like the barbarian's rage.
Better yet these new rampage bears are the mounts for the Druid Devils that populate the twisted caverns and gullies. Rampage Bears with red steel barding ridden by Druid Devils and cursed wrath lances guard the fallen celestial.
When damage is taken by any of the party have them make a will power save or they go into a rampage as well. They attack with rage the closest being to them (even if its a part member. I think umberhulks have this kind of thing if you need reference.)
Maybe those clerics or paladins of Justice can better resist the rampage. Or maybe their spells and/or combat are heightened in this plane.
Your fallen celestial lies chained in a foreboding clearing atop a low circle alter of runed stone. Numerous rusy chains run from the anchors in the stone to the fallen celestial. Clasped and riveted shut around waist, arm, ankle, neck, wrist, etc.
Of course they are not ordinary chains. You can hack em, but once severed they disappear only to form another. The chains are magical and of the celestial's own making. Each chain is a link in the psyche of the celestial to something that invokes his wrath.
He is unkempt, unwashed, and underfed and absolutelty red-eyed with anger. He communicates in harsh splutters between clenched teeth.
His redemption could come as the players must roleplay working the celestial through each of his 'issues'. If they can get him to pass Willpower saves on all his chains he is free to go. Or, maybe the players notice that when the celestial sleeps the chains wink in and out (or are gone, or grow weaker). Getting him of the circle of runed stones frees him.
If the players set foot on the circle of stone, they to run the chance of creating chains of their own.
Take it for what you will. Just mindless ramblings. Sounds like fun to me to fight like heck against devils riding rampaging bears only to find the solution to the quest something you can't hack against. You have to use those other skill like diplomacy, bluff, persuasion, maybe even intimidate.
