Yep, whatever legendary, but as I said, abilities and forme is randomized. We want to cover all variables.
And, for me, end of round is after the moves have finished and any effects (statuses, etc.) have occurred. Then the legendaries change to whatever and THEN you apply any other effects, such as if I had a Shaymin that was burned, it would receive burn damage, then it would become a Heatran and be 'cured' of the burn, but it would have still received damage as the health and energy remain the same.

Hopefully you got all that, so randomize a legendary!

And I'll attack first.