My personal, dark idea of what the cluster could be like.
It's the screaming that's the worst, it's the screaming that gets to him, even as he cowers on the floor, feeling Peridot's footsteps next to him. The screaming.
It's loud, painfully loud, even though there are no visible mouths, the sound takes up the room that served as the Cluster's sleeping place. The smaller gem fusions he's seen sometimes screamed too, but this is worse, so much worse. The screaming is raspy, many voices combining into one mournful howl. The Cluster moves, writhing, and Steven cries.
There's so much anger, pain and sadness in that mind numbing screech, and maybe it's the worst, but the sight is bad, too. Different sized and colored hands and feet, eyes rolling every way, a torsos bent in half, necks with no head. The gems create sparking lights on the floor. The Cluster moves and cries in pain, it's existence an abomination, every moment more painful than the next. The screaming has no words, but Steven understands the message-"Kill me! Kill me now!"
Even as Steven crumbles to the floor, he screams, begging for Garnet, Pearl, Amethyst, his mother, anyone. Peridot's hands are on his shoulder, and he wrenches for her grip. She knew this was happening, why did she help, why didn't she care? He runs from her.
The Cluster is constantly shifting, pulling, stretching into new forms. At on point, Steven sees a horribly misshapen face look towards him, a look of anguish on it, before it twists and morphs into something else. He tries not to look at it, but he can't pull his eyes away. The Cluster moans, and screams, and he wants it to stop.
He notes that the Cluster is slowly increasing in size, coming closer and closer to him. A dull yellow hand stretches and slams against the room's ceiling, making the room shake, and Steven wonders if that just caused an earthquake in Japan. He yells, wordless, just letting noise come out. Snot and tears run down his face. A pale blue-green foot swings toward him, and he runs away, tripping over his feet. He's scared, so scared. He wants protection, he wants comfort, but mostly he just wants the screaming to stop.