Latias/Latios

Latias/Latios

After beating the game, you can head over to the Dreamyard in the southern part of Unova to find either Latios (Black 2) or Latias (White 2), who will want you to chase them all over the Dreamyard in order to catch them.

The full instructions for this are covered in the Dreamyard section of this walkthrough.

After chasing them down, you'll have a chance to battle them!

Dreamyard
Latios Latios  Lv. 68
Black 2 only.
Latias Latias  Lv. 68
White 2 only.