Eeeh, I would have prefered an ex-machina character to show up. This one isn't even foreshadowed. Like, how does he or anyone even know time travel magic? It's too much of a coincidence. At least if some stupid crazy strong character showed up out of the blue, it can be excused at their curiosity on the abundant mana coming from the enemy, and can later lead to MC either learning to be stronger with a grander scope of what kind of power he needs.