Neothelids were created by Wizards of the Coast in the 2nd Edition of Dungeons & Dragons RPG in 1998 and are now part of the Forgotten Realms lore. Where they were colossal mutated offspring of the Illithids (aka, Mind Flayers) who were hated by all. They became part of the Cthulhu Mythos in Paizo Inc's Pathfinder RPG in 2008, with the earliest mention being in the Darklands sourcebook, in which they were made the spawn of Shub-Niggurath.
Neothelids are gargantuan creatures with powerful psionic abilities. Which gives them the ability to teleport on the battlefield, communicate with others telepathically, move objects telekinetically, and fly without wings. Their minds are strong enough to overwhelm weaker minds, sometimes fatally, and influence those with weak wills. Neothelids have the ability to ascertain where a target is going to teleport and thereby pursue said target. Physically they are superhumanely strong and have hides which can resist all blows except those of cold iron weapons, in addition to shrugging off spells cast by all but the very powerful. From their maws they exhale a cone of acid at their enemies and within are four barbed tongues which can inflict wounds as well as drag their prey into their mouths which can easily swallow smaller creatures. Biologically they are immortal but their Neothelid eggs centuries to hatch though they can spawn hundreds of seugathi each month, creating armies within decades
It's a race of powerful and foul worm-like creatures, that are cunning and intelligent. They are massive worms with four barbed tongues, drenched in slime. With incredibly tough hide and whose maws easily swallow smaller creatures. Millions were spawned on Golarion by Shub-Niggurath during the Age of Creation. Who then borrowed deep into the Darklands, establishing a great subterranean empire where they hunted and consumed each other for sport for 10s of thousands of years. When other races arrived in the Darklands, they were overwhelmed by these races which bred at faster rates. Which forced them to retreat, stopped hunting each other, and reproduce. Centuries later they began to expand their territory, and are currently waging proxy war with the intellect devourers. hey usually live alone and when they rarely do gather, it's in small groups. Seugathi offspring are used as servants, their strongest as bodyguards, and the seugathi speak on behalf of the Neothelids.