Time Slime is a demon from the Chest of Demons with the power to control the flow of time.
Time Slime is a tall, cadaverous man with pale green skin, a bald head and blue goggles. He also wears a blue cassock with a broad collar and a clock insignia on the chest.
Time Slime is as ambitious as he is tyrannical. His objective of world conquest seems only for the sake of defiling the world and having everyone under his rule. He is abusive to his subordinates (he browbeats Boogle and Weerd, even more so than the other demons). He is sadistic and has a penchant for designing elaborate demises for his enemies, which is his undoing -- when Scooby, infused with an uncharacteristic bout of courage, attacked and destroyed his trap, causing it to backfire.
The 13 Ghosts of Scooby-Doo: "Slime Time"
Scooby got a view of the future that would happen if he quit and let Time Slime win. As Vincent Van Ghoul said, "Everything went wrong." This is an understatement -- the world in the future is a post-apocalyptic junkyard, with very few people left (or at least wandering around). Time Slime seems to now rule the world, but he doesn't seem to be doing much with it but defiling it. Flim-Flam (now called Slime-X) and Scrappy have become disciples of Time Slime and wander the wasteland on motorcycles looking for victims to torment. Daphne has been enslaved and is forced into endless pointless labor (cleaning a dungeon). Oddly, she says that she enjoys working for Time Slime, but it is also apparent her sanity is severely eroded. On the other hand, Shaggy has gone completely insane, believing fervently his old friend Scooby will come back and somehow turn this horrible world around -- in fact, this seems to be the only coherent thought his mind can contain at this point.
How far this alternate reality is in the future is never made clear. Scrappy and Flim Flam still seem to be pre-teens, but Daphne and Shaggy both look 20 years older (this of course could be due to this nightmare existence causing them to age prematurely). (It's A Wonderful Scoob)