They are in Snowpoint City like in Diamond and Pearl.
The Solaceon Ruins.
Arceus
I think you have to find a special pokemon
lol there are no eterna ruins that was just in the anime.
in the solaceon ruins.
you get it at the solaceon ruins at the end of it
you go in from the veilstone way
Unown. In the solaceon ruins
You can find the M form of Unown in the Solaceon Ruins of Pokemon Platinum. This form of Unown appears in the dead-end rooms of the Ruins.
in the socalen or what ever ruins
its in the ruins of solance town.
in a house near the ruins