Which DLC are you talking about Cleglaw? Crucible? The AoM expansion?
Crucible is a random arena DLC where you fight waves of foes to earn loot and challenge your build basically. You start at 0 wave and can work up to 170; a wave is 10 at a time so 0 to 10 then stop, talk to Lokaar to decide whether to continue or get loot (if you do you start back at 0 again). If you die you also have the choice of collecting loot before beginning again. The thing is the higher you get your level the better loot you'll get so if you succeed at 10 it's better to push on to 20, then to 30, etc, for better rewards. 3 difficulties Aspirant, Challenger and Gladiator. It's good for farming runs I understand, but personally I find it boring and don't play it.
https://store.steampowered.com/app/483840/Grim_Dawn__Crucible_Mode_DLC/The AoM expansion adds another 2 acts to the main campaign and also adds the Inquisitor and Necromancer masteries along with new heroes/bosses, loot, factions, etc. Yes, it's definitely worth getting. Unlike TQIT/AE where you have to play the expansion to unlock the next difficulty Crate have made it so once you kill Loghorrean the next difficulty is unlocked so you can skip the expansion on lower difficulties if you want to.
https://store.steampowered.com/app/642280/Grim_Dawn__Ashes_of_Malmouth_Expansion/Bear in mind that there's a 2nd expansion in the works which should be released later this year and it will require you to have AoM installed to be able to play it.
You'd need to post in the GDDefiler thread over on the GD forum to see what Soul has to say. I've never used it for anything other than seeing where my toons are ingame so not sure what's working and what isn't yet.