The Brethren Moons are one creepy concept. I've always liked the idea of sentient astronomical objects quite a bit, just this massive thing that is harboring a huge secret. LOCAL58 pulled off this type of thing really well. In addition to looking terrifying, what with their writhing tentacles and maw at the center of their mass, but the sounds they emit are pure nightmare fuel, bone-chilling squishing and ear-piercing squeals just produces such a scowl-inducing effect in me. It's something you want to stay just extremely far away from. You know, it's a shame these things only appear in Dead Space 3 (besides technically being background antagonists in the first two) which relegates to them to obscurity, as I believe these moons have a ton of potential in crafting further intriguing horror scenarios. Discovering they were behind the Markers was a huge deal and Dead Space 3 DLC ends on a cliffhanger, so unfortunately we may never know what becomes of these gigantic, hulking behemoths due to Dead Space basically being a dead-in-the-water series at this point...
It's a shame we'll likely never see more of the Necromorph and Brethren Moon mythos.
The idea that they're the last stage of necromorph evolution and that they're the reason humanity has never found any alien life as they ate it all is strong.