Hi! To get to the [SPOILERS] Underworld, you’ll have to do two things: firstly, you’ll have to climb up the cliff behind the Beacon/Lighthouse (go up/left) and find the Needle (which is in a cave in the cliff face up there). After that, you just have to enter the lighthouse with the needle, and a portal to the Underworld will be there! [SPOILERS]
If you want a smaller hint, talk to Ludos, the lighthouse keeper dog, a few times. He’ll generally direct you toward your main objectives in this portion of the game, and even after this he can give hints, as well as other flavour text. I’m sorry for getting you stuck in my game, and I hope this helps!