Project Overlord was originally the second major DLC, and it’s about Ceberus, and their project to control the Geth. The Legendary edition already has it, so it doesn’t need to be purchased.
Choice: Take David or not
At the end of the quest line, you’ll have a choice concerning whether to bring David back with you or leaving him with his brother Gavin for the project.
If you decide to bring David back with you, you’ll take him to Grissom Academy, which results in you seeing David in the Mission: Grissom Academy: Emergency Evacuations in Mass Effect 3. When speaking to him there, he’ll give you access to a room containing a Sniper Rifle Piercing Mod as well as a new weapon.
If you decide to leave David with his brother, Gavin. Not only will you not get the mod and weapon, but you’ll find out from Gavin during the Mission: Arrae: Ex-Cerberus Scientists in Mass Effect 3 that David stopped responding from the stimuli, and had to be euthanized, poor David.