Where to find and activate the portal to Ember Valley (spoiler alert)
Where To Begin
that means feeding the Gordo slime 32 of any food. Once the Gordo slime pops, it will reveal a geyser that will shoot you to a ledge just above where the Gordo slime was.
There you should find an interactive device that will then open the portal to Ember Valley ahead of you.
I hope this helps! Personally I thought the portal would be located at Starlight Strand and I spent so much time combing through that area I almost went crazy lol