Dungeon events are typically theme or season specific. The new dungeon will appear in Time Rift and will remain open for a fixed duration.
Dungeons for these events typically have three “levels” matching the difficulties you already know (normal/hard/insane).
Those dungeon events also typically come with skill challenges and dungeon wins challenges (meaning the more runs you win, the more rewards you get).
You usually get an opportunity for a dungeon specific toon (by fragments earned in the levels) and come with associated fragments of typical souls (sometimes class specific)
The rewards also encompass rare talismans dropping randomly from the dungeon runs.
Rewards from winning the dungeons are usually much better than your typical Cosmos levels, but aside from the rare talismans and event specific fragments, won’t change your habits regarding the game.
So a few pieces of advice to run those :
If you’re looking for the fragments, try to run the madness difficulty
Aside from those, all other rewards are equally good and have the same drop chance in all three difficulties (so if you’re running for the talismans, do easy or hard)
Number of win goals can be achieved running normal difficulty, so remember that
Event specific toons usually have good sacrifice value, so look at the timeline, those fragments you get can transform into a horde of dupes for sacrifice