This intriguing DCOM is moving forwards, with cast now announced.

Kristen Chenoweth - Maleficent
Dove Cameron - Mal (daughter of Maleficent)
Maz Jobrani - Jafar
Booboo Stewart - Jay (son of Jafar)
Kathy Najimy - The Evil Queen
Sofia Carson - Evie (daughter of the Queen)
Wendy Raquel Robinson - Cruella DeVil
Cameron Boyce - Carlos (son of Cruella)

Here's the synopsis from when it was first announced.
In a present day idyllic kingdom, the benevolent teenaged son of the King and Queen (Beast and Belle from Disney’s iconic Beauty and the Beast) is poised to take the throne. His first proclamation: offer a chance at redemption to the trouble-making offspring of Cruella De Vil, Maleficent, the Evil Queen and Jafar who have been imprisoned on a forbidden island with all the other villains, sidekicks, evil step-mothers and step-sisters. These villainous descendants (Carlos, Mal, Evvie and Jay, respectively) are allowed into the kingdom to attend prep school alongside the offspring of iconic Disney heroes including Fairy Godmother, Sleeping Beauty, Rapunzel and Mulan. However, the evil teens face a dilemma. Should they follow in their nefarious parents’ footsteps and help all the villains regain power or embrace their innate goodness and save the kingdom? Kenny Ortega, the Emmy Award-winning director and choreographer of Disney’s record-settingHigh School Musical TV movies and feature film, returns to Disney Channel to direct Descendants.
The Villains In The Disney Descendants Movie Have Been Cast