Descent Champions, like Descent, is a special scenario separate from the storyline filled with challenging monsters. Unlike its predecessor, however, this version sports a choice in forks in the progression and a selection of unique items to pick from at the start the event. Purity has been changed so that it doesn't alter players' maximum resistances; instead players can utilize the new player purity aura gems for fire, ice, and lightning.
You can get more details on all these changes (plus a spoiler for one of the five new uniques coming!) in the update video below.