I picked up Van Richten's Guide to Ravenloft and have been reading it cover to cover. This is my review of the book, chapter by chapter. Chapter 3 covers over 30 domains, so I'm posting about it as I work my way through the different domains.
Hazlan is a vaguely Dark Sun domain, crumbling from the abuse of magic by its darklord Hazlik. Before he arrived in Ravenloft, Hazlik was a Red Wizard of Thay (in the 3e version, he was just a red wizard and his tattoos had been forcefully applied to him as a form of humiliation). As he'd risen in ranks in Thay, he developed a rivalry with another Red Wizard. He went to such extremes to gain power over his competitor that the zulkirs (high ranking Thayans) kicked him out. He ended up in Ravenloft with his own domain, where he reigns as darklord from his castle Veneficus.
Hazlan in 5e borrows a trait from the missing Azalin Rex. In 3e, part of Azalin Rex's curse was that he couldn't learn new magic. In this version, it's Hazlik that can't learn new spells.
What Hazlik is attempting, though, is to use his entire domain as a sort of spell focus or catalyst for greater magic or something. He aims to create a storm in a teacup (with magic being the storm, and Hazlan being the teacup) to get himself out of Ravenloft. He doesn't care that he's ruining his world, because he doesn't anticipate staying around when it finally implodes.
So he's a super-powerful guy causing all manner of ecological calamity with the fallback plan of leaving everything behind when he's used it up. I'm so glad this is just fantasy, because a villain like this in real life would be horrible.
This domain doesn't have just one plot, but it has a good plot table with lots of ideas for why player characters might have to spend some time in Hazlan. Hazlik, the Mulan population, the cities, the alien lodestone buried beloy Toyalis, and even Hazlik's castle, aren't the main attractions in Hazlan. The real excuse for dropping your players into Hazlan is the fact that magic doesn't work the same way in Hazlan as it does elsewhere. Due to the rampant abuse of magic there, all magic use results in a special Hazlan wild magic roll, and potentially a second roll on the wild magic table in the Player's Handbook (PHB). Any MacGuffin will do, and the wild magic is guaranteed to make it worth it.