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 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 School that has until then only ever housed the children of heroes. A culture clash ensues.
This first film was released in 2015, and was followed by an animated web-television series, books, music albums, and more.
In 2017 a second film was released, 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. 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. She also does a nifty final act homage to the TLM film's 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 2017 tie-in book Rise of the Isle of the Lost by Melissa de la Cruz, which features other TLM-related elements.
The events of this book take place just before Descendants 2, and it 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 Ursula'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 was smashed during Eric's final battle with Ursula.)
While gearing up for Descendants 3, a short film with Mal and Uma titled Under the Sea aired in 2018. It features an underwater encounter between Mal and Uma, and a cover of Kelly Clarkson's Stronger.
A third Descendants film was released in 2019. Uma and her gang make a return, but this time they have to work together with the original four Villain Kids Mal, Evie, Carlos and Jay, as well as their new friends in Auradon Prep, against an unexpected enemy.
Uma especially has to (re)learn to get along with Mal and combine their magical powers. The conflict comes from how Uma feels responsible for the people of the isle, and is angered that Mal seems to have forgotten her roots and isn't doing what she promised, i.e. bringing more people through the magical barrier into Auradon.