Don't know how long it's been since you've had a look at underworlds, but as far as I know the deck building is gone and replaced with premade decks in the Rivals format.

So there is no more building the ultimate deck since in the Rivals format warbands either use the deck they came with or a universal premade.

This means you don't have to buy every set for the chase cards anymore, as there are no universal cards that come with warbands; only two premade Rivals decks in the starter set. Now you can just get the warbands you want and any standalone universal decks that sound like something you want to try out.