
If you're looking for manga similar to Tales of Luminaria: The Fateful Crossroad, you might like these titles.
In a strange twist on Arthurian legend, young Art pulls the magical sword Excalibur out of a stone--only to find himself as one of a million Arthurs, fighting to defend the island of Britain from enemies and to earn the rank of the true king. Art is pretty puny, but his childhood friend Ligu (who was also chosen by an Excalibur) tries to keep him from being immediately killed. Soon Art lives in a castle with knights, which are created beings who serve the kings. From the respect with which he treats the knights, he inspires some to hope he becomes the true king. But is that really achievable by a scrawny crybaby like him?
In a land of swords and magic, there was a creature—an evil god—with extraordinary power. People feared its existence, but at the same time were captivated by its power. They wanted that power. Their dangerous wish would eventually lead them to break the evil god’s horn and create “Shin Kaku Gi Kou,” huge magical weapons infused with the horn. One of these weapons alone has enough strength to overpower an entire country and its army, and there are 10 of these across the vast continent. However, one day a severely injured Moebius, the strongest Shin Kaku Gi Kou user, entrusted his “power” to a young boy named Miyabi who happened to be present.
The manga is an anthology story centering on the Crypters, the betrayer agents of Chaldea that are the center of the Fate/Grand Order smartphone game's "Lostbelt" story line.