The Hummingbird Egg, also known as the Egg of Light, is a key item in Mother 3. It holds the memories of everyone in Tazmily Village before the destruction 100 years prior to the story.
The Hummingbird Egg first appears when Wess, Duster, and Kumatora infiltrate the Osohe Castle. This is the true item Duster was supposed to steal.
After finding it, Pigmasks appear behind the door, and have Claymen try and break the door down. Kumatora grabs the egg, initiating a trap. This leads to the party fighting the Oh-So Snake.
After the snake's defeat, the three, with the egg, are flushed down because Fassad had Salsa pull a lever to activate the drain. Wess and Kumatora appear washed up by Wess' house, but Duster and the egg both disappeared.
You later find it at the Thunder Tower in Chapter 4. A Clayman is holding it at a place nearby, but someone takes it to the Garbage Dump. You will have to fight a stronger Clayman than the original to obtain the Hummingbird Egg, called the Forlorn Junk Heap.