Shingeki no Kyojin - Before the Fall

The story follows the creator and the creation of the 3DMG, Ángel Aaltonen. A mysterious youth named Kyklop and various other characters. Set around 70-80 years before the main story of Eren & Co. In the year 770 to 790 of AoT's fictional alternative world.

Note: The author is Shiki Satoshi, because he's the one adapting it from the novel. Isayama Hajime is the series's original author, even if he has some involvement in this manga, he's still not the one drafting it.