After you beat the game, go to Rands house. He and Leanne
will ask you to obtain a book called "the forbidden song".
You can find it on the 3rd bookshelf in the mansions basement.
After that, take the book back to Leanne and she will read
the song. A boy from the past will tell you that Latias isnt
the only one in Oblivia. Then fly to Tilkule monument and walk up
to the stone tablet. A scene will occour and Latios will attack.
Capture it and you get a quest clear.
Hope I helped
For seriously, call me Iams or Darkrai please ^^
Currently training for the VGC.
I own many event pokemon from all events other than dream world ones, if you missed an event I can work out a deal with you =^^=