If there are only local units in the arena, how does Magma Wyrm move and attack?
If all units in the arena are local, all rows are considered to have the same number of units, 0. So then, we move on to the tie breaker, and it moves to the row with the most player-controlled units among those tied rows. If there’s a tie here, the player decides. Then, whether local or not, the Magma Wyrm attacks all of the units in its row.