The Lost Legacy is a spin-off from the Uncharted series following series regular Chloe Frazer and mercenary Nadine Ross featured in Uncharted 4. Rather than a simple DLC mission the Lost Legacy is a full-length Uncharted adventure with all the production quality you would expect from a Naughty Dog game.
Some more info via Wikipedia: “Uncharted: The Lost Legacy is an action-adventure game developed by Naughty Dog and published by Sony Interactive Entertainment in August 2017 for the PlayStation 4. It is a standalone expansion to Uncharted 4, and the first Uncharted game not to feature protagonist Nathan Drake. Players control fortune hunter Chloe Frazer, who seeks the Tusk of Ganesh in the mountains of India in the midst of a civil war, with the help of ex-mercenary Nadine Ross and Nathan’s brother Sam Drake.”