Other CR 20 fiends plot and plan and scheme. Not the olethrodaemons. They just want to cause death and devour souls on a mass scale. (See, kids? It’s important to have goals.)
To me, olethrodaemons are interesting because they are by definition juggernauts—that is, practically living siege towers. So I also imagine things riding on them or accompanying them: other daemons, fiendish vermin, or undead basking in the fumes of the olethrodaemons’ soul-drained breath.
And, of course, there’s the question of whom these daemons serve: Is it a Horseman, a mad mortal, or their own ends? Which leads us to another interesting thing: Olethrodaemons are actually comparatively risk-free to summon compared to most other fiends (and even certain super-celestials like solars). These daemons want to be summoned; they want to cause epic destruction. So they’re actually (as Horseman of the Apocalypse and Ultimate Magic point out) practically bargain-priced as forces of annihilation go.
Oh, and PS: Four stomach chambers. That drain levels. Getting swallowed by one of these things? Sucks.
The city of Preservation is one of the only safe refuges for adventurers on the Plane of Daemonus. No lesser daemon will travel near it, as Preservation is founded on the site where the Oinodaemon once dewinged and beheaded an early Horseman of War. But now an olethrodaemon bears down on the city like an engine of death, the call of the many succulent souls there too strong to bear any longer.
An olethrodaemon is summoned to destroy the dam at Bighorn Pass. A creature of fiery hunger, this daemon favors its wall of fire attack, and fiendish magma elementals and whole flocks of fire mephits drop off its bloated body as it travels. While these parasites are not much of a threat to adventurers who can stand up to an olethrodaemon, the confusion and devastation they add to the encounter further confound rescue efforts.
Hemisite is an olethrodaemon who serves the Horseman of War; hunts of sangudaemons (see HotA) ride its back. The olethrodaemon paragon Um Kul Tar serves the Horseman of Pestilence and revels in destroying cities already weakened by plague. The bone-white orb Skullcrusher serves no one, but it leads two devourers on a chain and once was seen vomiting up a dread wraith.
—Pathfinder Bestiary 2 70–71
As usual, I avoid mentioning planes that are Paizo IP by name above. Obviously most Pathfinder campaigns will locate their olethrodaemons in Abaddon; in 3.5’s Great Wheel they hang out in the Gray Wastes and especially Gehenna. In fact, the crawling cities of Gehenna might actually be olethrodaemons grown to enormous size.
Also, the olethrodaemon should not be confused with the olestradaemon. The latter has only one special attack, but it’s a doozy. (*rimshot* Thanks, folks; I’ll be here all night.)
Still encouraging reader questions and feedback this week. Send me an email—to avoid spambots, I’ll type my address as dailybestiary [at] gmail [dot] com, but you get the idea—or leave a comment and I’ll do my best to respond here.