just a disclaimer here, I haven't read ahead (if the story's even available in another format with more progression I don't know about it) so this is NOT spoilers, just me musing about the story so far and conjecture about last boss guy's nature...also it's pretty wordy and could be entirely wrong if I misremembered/overlooked something, it's been a while since I did a full read through...
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so I get the whole he's not the final boss thing has been stated before, but so far there's been repeated evidence he's the same type of existence as the final boss...which makes me wonder just how many cycles of time have passed? presumably this should be at LEAST the third timeline if that is the case, since original cycle (presumably very bad things happened) --> more cycles? --> MANY more cycles? --> cycle MC interrupted --> current cycle
and given what we've been told about time travel it kinda makes sense too, after all it's supposed to be super hard to travel back in time, with one strong mage's mana being worth less than a second (or alternatively the ritualized sacrifice of HALF THE HUMAN RACE only buys you enough mana for a couple years time jump), but in theory, if you could start the war and build a mountain of sacrifices in less than the time you gain by doing so, you'd be able to slowly but steadily travel back in time and presumably get stronger each time due to the soul fusing effect (and maybe jump just a little bit further each time as that gives more power and experience+more power presumably lets you rerun the process that much more efficiently)...barring the luck of some random interloper (aka MC) you'd be able to travel at least the full duration of your life, which if last boss is a demon to begin with might be centuries or even millennia, it'd probably take hundreds if not thousands of cycles, and you'd pretty much have to be nuts at the start to not go insane from all the time jump stacking memories of the same people over and over and over and over... and the soul corruption that results from the fusing, but eventually you'd be able to undo something you truly regretted
also, technically speaking, if he actually succeeded, he might even come off as a decent person in the end, because the time travel jump > time of slaughter, he'd have not caused any lasting harm to anyone except himself (since everything but him gets reset again with the final time skip)
now I realize that would in turn beg the question of why said baddie is still up to no good in THIS time line, and to that the simple answer is, since the spell wasn't actually ready yet but accidentally got fired off by MC after the fight, it wasn't at full strength and thus put him AFTER the point last boss reset to in that cycle
that also indirectly explains where the dragon heart mana crystal thing came from despite no one currently knowing what happened to it, it was found who knows how many decades/centuries into the future and as both the focus point of the spell and power source it got sent back with last boss through the resets (sure he could have found it in the more recent future and just no one knew he was looking/where it was and that part was just luck, but that seems too random to fit given what all else this dude managed so far without even showing up in person)