Obsidian is a fusion in Steven Universe of Steven, Garnet, Amethyst, Pearl, and by extension Ruby and Sapphire. Obsidian is seen only once in the series, in Change Your Mind, Steven Universe S5E29. She is fused to get into the Diamond Mech, and talk to White Diamond. She only speaks in primal roars in the appearance, it is currently unknown if she can speak. She combines all of her components weapons into a hilt of a greatsword, and forges the blade in a third mouth full of lava, breaking the theme of literally combining weapons in fusions. Her voice actor is all of her components' overlayed.
Obsidian combines the arms of her components to have eight. One of her hands has Steven's clothing on its fingers. She has two faces, three mouths, and four visible eyes, with more possibly under her second face. Her eyes have flames instead of eyeballs. Her skin is gray, with line designs of lava across her body and limbs. Her knee has a triangular protrusion, and her feet have a triangular indent. Her hair is black with rings, lines, and spikes of lava. She has all of her components gemstones in black. It's assumed that the temple that the gems live in is a statue of her, but a version of her with Rose Quartz instead of Steven.
In an effort to ascend the complete Diamond Mech, Steven suggests that the four core members of the Crystal Gems should fuse. The other Gems agree, and Garnet declares that it is time to form Obsidian. Garnet, Steven, Pearl, and Amethyst soon perform a fusion dance before coming together to form Obsidian. As Obsidian lets out a tumultuous roar and the other Crystal Gems express their awe in seeing Obsidian, the Diamond Mech soon returns. Obsidian begins to climb up the mech briskly while Connie, Bismuth, Peridot, and Lapis distract the upper part of the mech.
After ascending most of the mech, Obsidian begins to summon their weapon, a gigantic greatsword. Doing so leads to the attack of the Diamond Mech, managing to slice the wrist of the mech's right arm, and in half for the left arm. They then proceed to immediately thrust their sword into the mech's chest, and Obsidian then leaps onto the mech's face, roaring into its eye. Thrown off-balance, the Diamond Mech proceeds to tumble throughout the Homeworld city, ramming its face into a pillar and unfusing Obsidian.