Egg

The Egg is a mysterious Key Item that can only be found when not a single EXP was obtained throughout the game. There is currently one Egg for each section of the game. The Egg is required in order to obtain a Section's secret item. Attempting to take an Egg outside of its respective section will result in it disappearing from your inventory.

Egg locations and rewards

As stated above, in order to obtain each section's respective Egg, not a single EXP must have been obtained during the game from any source, including killing the section's final boss or making any enemy flee, thus requiring a full Pacifist Route. Additionally for Section 3, you also need to battle and spare every single enemy, not including Jerry.

After obtaining an Egg, it can be placed into a specific location to obtain its respective reward.

Section 1 - In Ruins

Egg's location

• Head to the guest room in Toriel's home to sleep and watch the dream sequence.
• Once you wake up, make your way back to the Hall of Independence to find Migosp there (in Hard Mode, Migosp is replaced by Migospel).
• Interact with them and they will give you the Egg.

Egg's reward

• In the room with the Monster Candies, you can put the Egg in the bowl to exchange it for the Chocolate Candy.

Section 2 - Oh, Mother

Egg's location

• After Mondo Mole's fight, a little mole will start following your party around.
• With the mole in your party, make your way towards the Hint Man. He is located in the left direction of the cave where you first appear in Section 2.
• If you speak to the Hint Man with the mole in your party, he is grateful for you bringing his favorite. Afterwards, he will give you the Egg.

Egg's reward

• In Happy Happy Village, interact with the picnic stand to put the Egg into the Boiled Eggs bowl; shortly after it will be traded for the Aluminum Bat.

Section 3 - Hell's Forest / Snowdin Intermission

Egg's location

• Battle and spare every single enemy in Section 3 (with the only exception being Jerry) before jumping through the grey door. A few enemies are easy to miss such as: 2 Chilldrakes at the sentry stations of Dogamy and Dogaressa (Near the room with the Ball Game), an Ice Cap inside the hole of the Ball Game, and 3 Ice Caps hiding in a snow poff before the Greater Dog fight.
• After jumping through the grey door in Section 3 and appearing in Snowdin Town's Inn, go upstairs to find the Librarian NPC from Deltarune. They only appear if the above condition was met. Once you interact with the NPC, they will give you the Egg.

Egg's reward

• Head west of Snowdin Town and meet Papyrus (Undertale), and he'll trade the Egg for the Papyrus's Charm (AKA Papyrus Charm)

Trivia

• When speaking to Migosp/Migospel and Hint Man to get the Eggs, they will mention that they both got them from a strange man.
• When trying to sell Section 3's Egg to Sans, he'll say "you should give this to papyrus", giving a hint on how to get the Papyrus's Charm.
• When talking to the Librarian NPC for the 7th time, they'll say if "You found this where you shouldn't be looking", that they can only imagine what kind of damnation awaits them, and that "After all, he watches us all the same" (they were most likely referring to Gaster), before disappearing.