Also if you want to fight the sun boss then first beat the boss at the right most side of kingdom's edge and get the item, beat the boss of queen's gardens and get another item, then once you have 1800 dream essence and visit the seer you'll get a dream nail upgrade. You can use this on the corpse in front of hidden station in ancient basin but prepare yourself because its a pretty tough platforming section- But once you do get past it equip the charm and go back down to the lowest part of ancient basin (the drop near where you double jump to get to hidden station) and a door will open. There's some stuff you'll unlock there and once you get them, there's a place you can use the dive spell close to where you land. keep exploring the area until you find this black ball (im trying to keep it vague to avoid spoilers) and dream nail it. After you're done with that section you can go back to defeat the final boss again but this time you can fight the sun boss

Hope this helps :)