The latest downloadable quest to join Shin Megami Tensei IV is “The Eternal Youth”. The quest costs $2.49 and is another Challenge Quest related to the main story.
In The Eternal Youth, your job is to defeat the Eternal Youth, Sanat Kumara. Defeating him lets you fuse the demon Chaos Sanat.
Atlus recommending waiting until you’ve completed the game to take on the quest. You can find screenshots from the Japanese version of the quest here.