https://eternalreturn.fandom.com/wiki/Damage
The formula is the same as League of Legends i.e. def_multiplier := 100 / (100 + defense), meaning there's no dimishing returns but in terms of effective health; I recommend checking out this article
https://leagueoflegends.fandom.com/wiki/...king_Armor
The main difference is that Eternal Return has a lot of damage that ignores defense (extra skill damage, extra basic attack damage), which makes mixing HP+Defense (or sometimes just HP) much m