Slayer edition was not DLC - it was Arrowhead’s complete revamp of the 2014 release, I think in response to player feedback. My understanding is savegames between the two aren’t compatible due to the extensive changes.
Gamepedia wiki details the original 2014 Gauntlet release:
The masteries system—which I’d call a sort of hybrid between an achievement system and a passive skills/bonuses/“chance to X” system was dropped for Slayer, I assume for simplicity:
The wiki’s Realms page explains a bit the ‘hub’ of the original edition vs. Slayer’s changes to that:
This Steam game guide details the differences between versions:
I found the original release too difficult for my tastes and the masteries system a bit tough to grasp. The original 2014 release version has its hard core fans, who apparently figure out ways to still play it, though I’ve ultimately found Slayer edition more to my liking.