get dialga palkia and giratina put them first in your party go in there and go in front of the painting and press a and a door opens up
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Take of the phyduck
In the cave or in one of the houses.
Celestic town shrine
you need azelf,uxie,mesprit to open the cave after beating the Pokemon league
you need four to obtain the HM, five to use it. You get it when you go into the cave at Celestic town and observe the painting