I just read the webtoon, and let me tell you, it messed up my timeline real bad... thankfully though, the webtoon covers 1000 - 1521 CE, so my timeline before was not messed up.
one thing that I was surprised about though that in the webtoon, there's an issue that talks about the deviants using cosmic energy gathered from the humans(or at least that's what I thought, because the deviant 'ate' a human). so technically, my imagination was canon all along lol.
list of travels from Eternals 500-year war:
-1084 CE, Heian-Kyo Japan: In here, Druig and Makkari fought a deviant that looks like a kappa, they talked to Yoshiee Minamoto.
-1142 CE, Korea: In here, Sprite and Kingo encountered a deviant that absorbed humans to get more powers. the deviant was shaped like a nine-tails(I think?) and Sprite was able to defeat it by trapping the deviant in an illusion, and destroyed the cosmic energy that the deviant was holding. one thing to note in this travel, is that Sprite could tell hundreds of years of history in a blink of an eye using a dream like trance, she told the story of the koreans to a man named bu-sik kim(I think?).
-1236, Iberia: In here, Phastos and Ajak met King Ferdinand III, they fought some deviants in the middle of the town, king Ferdinand thought that the deviants was the result of some necromancer's experiments(I think?). this issue was quite boring.
-1520 CE, South Atlantic Sea: in here, Thena and Gilgamesh was travelling with a ship when they encountered a deep sea deviant. it was quite cool to see Gilgamesh and Thena fought in deep ocean, and in here they realized that Deviants were stronger in the ocean, as they had a chance to develop. in this issue, they also talked about Ajak's worries about gunpowder.
-1500s(There's no exact date), Ming China: In here, Ikaris and Sersi fought deviants. they encountered a young white eastern dragon(I'm not kidding), and the dragon was killed by the deviants. after that, Ikaris slaughtered a bunch of deviants in a cave, but they were cornered, but then the rest of the eternals arrived, helping Ikaris and Sersi slaughtering the deviants. in here they also talked about the mental connection of deviants, turns out, a deviant absorbed some of Druig's cosmic energy and got some of his power, and they spread it to the others(I don't really get this one).
After that, they went on a world survey to check if there are deviants left. (I think this is just before the split in that Aztec city.)