Red Slime, also known as red liquid, is a gimmick that appears in Shadow the Hedgehog. It is a substance that serves as the Black Arms' main transport system inside the Black Comet and their colonies.
The Red Slime is a thick red liquid that runs along surfaces like some sort of vine. It serves as the high-speed transportation network for the members of the Black Arms species or those that are kin to them. When utilizing the Red Slime, the users absorb themselves inside of it and slither through their vines to the other end, which is marked by a Red Slime pool.
In Shadow the Hedgehog, the player can use the Red Slime to transport Shadow through its vines to different sections of a Stage, some of which cannot be accessed by any other means. Almost every Stage in Shadow the Hedgehog features Red Slime for transport. To use the Red Slime in gameplay, the player has to use the Dark Spin Dash when standing close to any trace of Red Slime.
In other media
In the Sonic the Hedgehog comic series and its spin-offs published by Archie Comics, the Red Slime is referred to as Conduit Gel on Sonic's World. It is described as an oozing fungus used exclusively by Black Arms for quick transport. It was planted on Sonic's World during Black Doom's attempt to invade the planet, and was also used onboard the New Black Comet.