Nintendo announced the Fire Emblem: Three Houses 'Cindered Shadows' earlier this week, and some new details have come in. First up is official info straight from Nintendo.
The story introduces new characters and takes place in an area below Garreg Mach known as the Abyss
The DLC can be accessed from the main menu in a different save file from the main story
The course of events will be different from other stories
Does not seem to use the main story calendar system
The battles are carried out with squads of predetermined characters, allowing normally incompatible characters to fight on the same side (e.g. Edelgard and Hilda)
The maps of this DLC feature new mechanics and tricks
This story is also linked to the main story, and when beating the DLC players can recruit the new characters Yuri, Hapi, Balthus and Constance to continue with the main story.
On top of that, there's another round of potential info that comes from dataminers. As long as you don't mind spoilers, check out the two videos below.