Demodands are an obscure race of fiend that originated in the Dungeons & Dragons second Monster Manual as a grid-fill, for Tarterus. Every lower plane needed its signature fiend (except apparently Gehenna/The Gray Wastes, depending on which of those two the Yugoloth are native to in the edition you're using), and that's what Tarterus got.
There are three sorts: farastu whose skin is tarry, kelubar who are slimy, and shator called "shaggy" but really very wrinkled.
In the Reign of Lorraine, "demodand" sounded too much like "demon" so 2e renamed them to Gehreleths. 2e made them the rulers/prisoners of their plane, now Carceri. But you wouldn't know it. Planescape detailed Carceri in "Planes of Conflict"; this had so much awesome - Vaath, the Anarchists, gautiere - that nobody even noticed the 'leths. And for fuck's sake, this was Tarterus. Hello? Titans?
2e had some lore about there only being 333 of each gehreleth sort at one time, that Apomps the Three-Sided created them all and gave them obsidian triangles. This made it so that nobody could cook up a fourth gehreleth, unlike devils and demons. Apomps needn't have bothered since nobody gives a shit about these demonish also-rans.
As their obscurity meant they were not protected, Pathfinder promptly stole them and used them for their own setting, reworking them as the bitter creations of the ambitious titans; failures to be what their creators wanted, the devout demodands seek to manipulate and force mortals into abandoning the gods to worship the titans instead.
In terms of what they do all day, do you remember that old mental image of the Christian Hell they use in cartoons, of demons in a fiery cave torturing and bullying sinners for shits-n-giggles? It's basically that, but much more metal and hardcore. Like, Crucifixion ain't got nothin' on the things the Ghereleth do to the sinners that come their way [also, fun fact, they animate their numbers from the abundant corpses this practice produces (the first generation of Ghereleth were allegedly animated from dead Yugoloth), so all three ranks actually have traits of both fiends and undead].
In obscure lore, it's stated that the Ghereleth race was ostracized at it's birth, which resulted in them declaring war on pretty much all the other fiendish races. Combine this with the fact that their meddling in the mortal world appears to be minimal compared with other fiends (the two examples used are Shaator A) influencing bards and poets into behavior no more harmful than producing works of Lovecraft-type cosmic nihilism, and B) going through the trouble of raising a primitive agrarian society into a vast wealthy empire just in case it produces any abusive nobles for them to torture after death), it could be argued that the presence of the Ghereleth in the Cosmos benefits Goodness and mortals more than it harms them.
|The Fiends of Pathfinder|
|Lawful:||Asuras - Devils - Rakshasas - Velstracs|
|Neutral:||Daemons - Divs - Sahkils|
|Chaotic:||Demodands - Demons - Qlippoth|
|Lords:||Ahriman - Archdevils |
Demon Lords - Four Horsemen
|The inhabitants of the Planes of Planescape|
|Upper Planes:||Aasimon - Angel - Animal Lord - Archon |
Asura - Eladrin - Guardinals - Lillend
|Middle Planes:||Formians - Githzerai - Inevitable - Marut |
Modron - Rilmani - Slaadi - Kamerel
|Lower Planes:||Alu-Fiend - Baatezu - Bladeling - Cambion |
Demodand - Erinyes - Hag - Hordling
Imp - Kyton - Loumara - Marilith - Obyrith
Succubus - Tanar'ri - Yugoloth
|Transitive Planes:||Astral Dreadnought - Githyanki|
|Inner Planes:||Azer - Elemental - Genie - Grue - Mephit |
Salamander - Sylph
|Sigil:||Dabus - Cranium Rat|
|High-ups:||Archangel - Archdevil - Archfey |
Archomental - Demon Prince