From PokéHeroes Wiki
Eternal Rock can be used to obtain an Eternatus egg.
How to find them
Eternal Rock can rarely be caught at the Emera Beach during a Thunderstorm.
How to use them
At the Ancient Cave you are able to summon an Eternatus egg if the Eternal Rock is in your Item Bag.
- Eternal Rock are tradeable.
- Hatching an Eternatus egg will count towards your shiny chain.