This can be obtained during the second part of the game or the "end game" after checking out the long cutscenes and ending up in the HQ.
Use your map a lot, it shows wherever you have unopened doors.
Steps:
1. OPTIONAL - Obtain the Magic Seashell. This is optional, because the route to the Sunken Shrine is always the same. It can be found underground in the Quillshroom Marsh. I think you need to go through the underground area first in the future and then in the past to obtain the Seashell.
2. Go to the Sunken Shrine by going down in the left-middle part of Howling Grotto. Once you get underwater, the Seashell will play a tune everytime you're near a correct opening. If you don't have the Seashell then just follow the below order. Once you get in the Maze through the straight part the order you need to swim is:
- Right (Upper)
- Down
- Down
- Right (Lower)
- Down (Left-most)
- Right
- Down
- Down
Now swim through the cave and you have reached the Sunken Shrine!
3. Complete Sunken Shrine. Simple platforming here. Once you reach the massive door with two sockets, take the upper-left route to obtain the Lightfoot Tabi, which allows you to run on water or lava. After that complete the bottom routes and you will receive the Key of Love and this achievement.