The Phantom Dimension can be accessed through a Phantom Portal item that players can put in their den. You can buy your own in the Saphire Shop.
Once in the Dimension, you can use the Paintseeds that you can collect from Paintseed trees to battle the Phantoms. Some Paintseed trees are available inside the Phantom Dimension, but if you would like more you can purchase in the Sapphire Shop for your den.
Each time you battle a Phantom and win you gain credit toward unlocking another area of the Phantom Dimension. But be careful, if you lose 3 times against a phantom it will send you back to the start!
You also gain Phantom Coins which can be used to purchase potions and other items inside the Phantom Dimension. Some areas have special hidden secrets, have fun and explore to see what you can find.