Chapter 3 missable achievement - once Ivo gives Wilbur the light and you see the lake, the 4 gods will each have a task for you. you only need to complete 3 to progress the story. completing the 4th one before moving across the lake will earn you this achievement.
the god of storytelling simply requires you to tell a story. you can choose any dialogue selections to make a successful story.
the god of riddles wants you to ask an unanswerable question. you must also answer his riddles first. the answers are Mountain, Teeth, Wind, and Time. you can ask your riddles in any order, and the 4th one will always be the correct one to gain his favor. (it's a paradoxical question, like "can you ask a question even you cannot answer?")
the god of humor wants a joke, and the god of the arts wants a musical instrument. after talking to them, make sure you grabbed the ring, then climb back out of the hole. talk to timmy and ethel, and ask them about jokes, then go through the pipe near ethel to return to the school. interact with the doll to get the headmaster's attention, who leaves after a conversation. now full size, take the glockenspiel, and other items and head down to ivo. give her the items and ask about jokes. she doesn't know any, but you can switch to her and ask the zombie and the two-headed ogre for a joke, then tell them to wilbur. now return to the gods.
give the glockenspiel to the god of the arts, and tell the god of humor the 4 jokes you learned, now tell him one of his own jokes. they will explain the secret of the lake after the 3rd task, but walk back to the gods and finish before you cross it. the first time you do, the gods will stop accepting favors.
thankfully, if you make a mistake, you are given the option to restart the chapter you are currently on. i thought i was going to have to start from the beginning, but when i pressed new game, i could choose chapter 1 or chapter 3 (but oddly not chapter 2).