
Spoiler (The emperor)
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Cronos's emperor is actually possessed by Aleksandr, the creator of aura in the Salamander Continent. He actually uses black magic to possess all new emperors (so he's been planning his continent conquest for centuries). So that's why Cronos could become this big of an empire (black magic + a lot of time).
If you want more spoilers about something in specific, you can tell me!

Spoiler (Deaths)
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To the point I've read, Henderson is the only one that was properly introduced as a character that has died. There's another guy that'll be introduced after Dimitri's first war against Cronos but he's not mentioned again until the the second continental war where he dies.
Obviously in every war, hundreds or thousands of his subordinates dies but not anyone relevant.
Oh, and recently someone who was part of Dimitri's army (someone quite important later on), was found out to be a spy and was killed by Kevin.

Spoilers
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They turning against Dimitri was a heavy blow but after the first continental war against Cronos, Dimitri prepared a lot for the off chance of another war against Cronos. Flora Lawrence became a great strategist for Dimitri and the leader of Haomun (?, Lucas, distrusts everyone, so they react smoothly to that guy's betrayal. He couldn't deal lots of damage before Kevin hunted him down.
I love op characters, it'd be dumb for someone to be compared to a god and then they don't live up to that. In other webtoons, the biggest problems other MCs face when living another life is growing their current selves.
In Roman's case, he was at the pinnacle and he could have ascended to heaven but he didn't, he already grew his past self so he is faster this time around.
The author of this story has already set The Heavenly Demon as someone close to being a martial god.
I'm currently on ch 387, and I can say this novel/webtoon isn't about Roman Dimitri himself (he already was the heavenly demon in his past life, he doesn't have a rival), but about him building Dimitri and conquering the continent. He can't do it alone and the people that follow him didn't have enough time to grow before Chronos decided to wage war, so it's more about Dimitri's struggle and success. With Roman's help or without his help they need to stand up for themselves (as proven on the last chapters I read, where Dimitri is struggling and fighting without his help, it's nerve-wracking though).
Spoiler
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Chrono's emperor isn't the final boss. I don't even know what's behind him, but we can expect them to be Roman's long awaited rival. Roman expects them to be powerful enough to fight him, because that'd mean he can finally feel the thrill of a good fight after a long, long time, since even in the Murim there was no one to put a fight against him.
There's also another big spoiler I can tell you if you want, about the usage of aura in the Salamander Continent.