Obsidian

"Garnet: We need to be bigger. Steven: Then let's fuse, all four of us! It's the only way we're gonna get up to her head! Amethyst: I'm in! Pearl: I'm in, too. Garnet: Then it's decided. It's time to form Obsidian."

Obsidian is the fusion between Garnet (by extension, Ruby and Sapphire), Amethyst, Pearl, and Steven Universe. They made their debut in "Change Your Mind".

Appearance
"They're huge!" Obsidian is a massive fusion with eight arms, gray skin, bright orange markings, glowing orange eyes, and charcoal-black hair that reaches the ground. They also flaunt a mask, which opens up to a third mouth. The third mouth is filled with lava Obsidian uses to craft their sword. Their appearance greatly resembles that of the Crystal Temple's hill figure.

Personality
Obsidian does not appear to have a distinct personality, as they are shown to be always silent. The only noise they make is a loud roar after their formation.

Abilities
Obsidian possesses standard Gem abilities and traits. As the fusion of all 4 core members of the Crystal Gems, Obsidian is the strongest fusion so far in the series.

Skillset

 * Greatsword Proficiency: Obsidian forms the hilt of their sword by combining Pearl's spear, Steven's shield, Garnet's gauntlet, and Amethyst's whip; then, Obsidian smelts the blade of the sword by opening up a third mouth full of lava and sticking the hilt into their mouth. The red-hot sword that they pull out of their mouth is then capable of slicing through nearly anything, as it destroys Yellow Diamond and Blue Diamond's ships with seeming ease.

Unique Abilities

 * Pyrokinesis: When Obsidian was eventually formed, their eyes (and in their mask) ignited fire.
 * Lava Generation: Obsidian's third mouth is filled with lava, which they use to craft their sword.

Trivia

 * The fact that Obsidian has the ability to create/hold lava within their body may be a reference to the fact that, in real life, obsidian a type of volcanic glass, formed when felsic lava cools.
 * The name of Obsidian was first seen in the Steven Universe: Art & Origins book.
 * Obsidian's sword's blade strongly resembles the Great Frostblade from The Legend of Zelda: Breath of the Wild, both in the blade's outline shape and the presence (but not exact design) of geometric designs on the blade's surface.