First fly to Nuvema town and head north to route u1. From there you can follow the imagine guides below to get to the Magmastone to start the quest. The Magmastone will be in a pokeball on the water. It’s marked with a Red BOX in the image below.
For the next part of the quest you will have to travel to Dr. Hold which is in a house on route u17. Use the image guide bellow to find your way to Dr. Hold.
After speaking with Dr. Hold you updated quest will now be to summon Heatran in Reversal Mountain. Exit the building and fly to Undella Town. Follow the image guide bellow to reach the area to summon Heatran. You will have to battle the Heatran which is level 50.
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After defeating Heatran the quest will update. Head back to Dr. Hold on u17. You can use the “Image guide from u18 > u17 > Dr. Hold’s House” above to find your way back to him. After speaking with Dr. Hold your quest is complete and you can now summon Heatran daily! Congrats!