During game setup, can you choose your battlefield and/or chosen champion after seeing your opponent's legend?

Ruling: You can choose your battlefield after seeing your opponent's legend, but your chosen champion cannot be changed - it is part of your deck registration and must be declared before seeing your opponent's deck.

Sequence:

• Step 1: Reveal legends
• Step 2: Separate chosen champion (already determined at deck registration)
• Step 3: Choose battlefield (based on what you now know about opponent)
• Step 4: Shuffle
• Step 5: Determine turn order
• Step 6: Draw 4 and mulligan

Nuances:

• In tournament play, your chosen champion must be the same for every game 1 of a match
• Between games in a best-of-3, you can sideboard your chosen champion from either your main deck or sideboard
• In casual play, these restrictions don't apply