The Disney Descendants franchise posits a magical, modern universe where various Disney stories took place at roughly the same time, and once the various villains were defeated, they were exiled by magic to the Island of the Lost. When the first Descendants film opens, we meet the next generation, i.e. the children of various known characters. The film mainly follows Mal, Evie, Carlos and Jay (the children of Maleficent, the Evil Queen, Cruella de Vil and Jafar, respectively) being invited by Ben, son of Belle and the former Beast, to join Auradon Prep, which has until then only ever housed the children of heroes. A culture clash ensues.
That first film launched the franchise, which now includes an animated web-television series, books, music albums, and more. In 2017 a second film joined in, featuring a The Little Mermaid-based character in a major role: Uma, the daughter of Ursula, who is also the film's main antagonist. Played by China Anne McClain, Uma is just as power-hungry and jealous as her mother, and has hyperfocused her rage on Mal, whom she believes has taken what could've, should've been hers.
Being trapped on the Isle of Lost, there's little that Uma can do but seethe and complain to the rest of her gang, two members of which are Harry and Gil, the sons of Captain Hook and Gaston respectively. Then opportunity strikes when Mal, who has been under stress due to her new role in Auradon, returns to the Isle, followed closely by her best friends Evie, Carlos and Jay, plus her boyfriend, Ben. Uma makes her move, and proves herself as wily and ruthless as her mother, in a nifty final act homage to the Vanessa subplot.
It's a pretty interesting world they've set up in this franchise (next gen stories are all the rage now, too), plus China's performance as Uma is energetic and captivating. On a personal level, Ursula and Maleficent are my favourite Disney villains, so there's a certain joy in having this universe's version of their offspring battling it out. Ursula herself has a cameo in the film, and though we don't see her face, she is voiced by the wonderful Whoopi Goldberg.
TLM-related characters appear in other parts of the franchise, too. Visitor/commenter Jay gave me a head's up (thanks!) about the tie-in book Rise of the Isle of the Lost by Melissa de la Cruz, that features other TLM-related characters and settings. The events of the book take place just before Descendants 2, and opens with Mal, Ben and their friends attending the Seaside Festival in front of Ariel and Eric's castle, and watching performances by Sebastian and other merfolk. Mal and her friends in particular befriend Arabella, who is King Triton's granddaughter and Ariel's niece.
Trouble brews when Arabella, too curious for her own good, "borrows" King Triton's trident and accidentally loses it in a storm. Bad timing leads to the trident being lost inside the magical barrier that surrounds the Isle of the Lost, and kicks off a race among the villians of the Isle to find the trident first. Among those hunting the trident is Uma, who over the course of the book wins her ship, gathers her crew, and pieces together Uma's seashell locket that had been smashed to smithereens. (Curiously, where the locket had been smashed due to Scuttle's actions in the film, in the Descendants universe it's smashed during Eric's final battle with Ursula.)