When you've finished all the boards in Pokémon Quest, you'll unlock Happenstance Island. These levels are very different from the previous ones, as they are time-dependent. You have to beat each level in 5 minutes, because otherwise you will lose. There are 12 levels, each consisting of 4 waves and a boss.
The higher the levels, the better the ingredients and the better the improvements you will gain. At Happenstance Island, you can get a lot of Mystical Shell so you can create legendary Pokémon that are powerful and will come in handy to defeat difficult opponents. To help you create the legendary Pokémon, we have created a recipes page and a Mystical Shell section in our guide.