The Star Spawn of Cthulhu (sometimes also named Cthulhi) are a race of monstrous creatures descending from Great Cthulhu himself. As such, merely looking at one can be very damaging for one sanity.
Origin :
As their name would imply, the Cthulhi could be the progeny of the Great Old One Cthulhu - unless they were just followers of the Great Dreamer who came with him from outer space. It is not really known how they were produced or their exact relation to Cthulhu, but there were legions of these horrors back when the Elder Things still roamed the Earth. War ensued until peace treaties were signed, granting them most of the landmass on Earth.
They're naturally extremely devoted to Great Cthulhu, and most likely originated from the corpse-city of R'lyeh. The first worshippers of Cthulhu may have also regarded them as heralds of their god. However, for unknown reasons, R'lyeh and many of the lands they owned sunk back into the seas. Still, these monsters still walk the Earth to this very day, awaiting their masters' return.
Appearence :
"What the hell could've spawned such an abomination ?"
(Detective Jack Walters, regarding the corpse of the creature)
As Cthulhu's offspring, the Cthulhi share a lot of his physical traits, such as an octopus head and a bulky grostesque form, though they seem to not possess wings. They're also not made out of ordinary matter - which could explain why bullets don't seem to phase them - and are said to possess some shape-shifting abilities.
However, they seem to be quite vulnerable to fire, as shown with Jack's method of getting read of one. They have a lumbering gait, which seems to show they are not very agile. They also possess a great physical strength, enough to lift a grown man effortlessly from the ground.
Lastly, their very form is so revolting that looking at one delivered a serious blow to Jack's sanity.
Role in the game :
"But who will be the enemy ? There has been no ructions with either the mi-go in the north, or Cthulhu's kin in the south. "
(Jack Walters during his time amongst the Great Race)
It is mentioned by the Yihtians that they fought against the Cthulhi before their civilisation collapsed.
Jack also encountered one of the Star Spawn guarding the backdoor to the old Masonic Hall during the raid on Innsmouth. Despite his warning, the soldier accompanying him is devoured by the monster, and Jack's firearms have no effect on it. However, he manages to use a flamethrower to burn it to death.
Trivia :
- In the book "At the Mountains of Madness", it is mentioned that the Elder Things fought against the spawn of Cthulhu, while the game states the Great Race of Yith also fought them. It could be a lore mixup, but considering how long the Star Spawn walked the Earth, it might be possible they fought against both races.