If I’m not wrong, the guy within Anastacius is the previous emperor who was deemed to be the “strongest magician” and who Lucas rejected to take under his wing when asked by his father. I speculate that he carried his feelings of resentment from being denied by Lucas and somehow became a dark magician kind-of thing who managed to keep his soul alive (maybe a demon? Considering that demons can live for a long time and possess others with dark thoughts) and corrupted Anastacius when Claude stabbed him.
Glad I could be of any assistance it makes sense that people forgot about these things given how often the translations are updated (bless your souls, you wonderful translators) but that just means more time for me to reread my manhwas and go over anything I may have missed after catching up. (What a geek, right?)

What up y'alllll so uh,if the guy (who is disguised as anastacius) knows anastacius' memories,does that mean they know each other?