What does the Byrdonis Egg do? Most of the time in Slay the Spire 2, you want to pick up items that help right now, because adding bulk to your deck often also adds problems. If you have cards you cannot use against an enemy, well, that can be a recipe for disaster. There are items in STS2, however, like the Byrdonis Egg, that can pay off big time later if you know what to do. All of the Slay the Spire 2 characters have a different playstyle, and you will want to build them in a certain way, but there are always some colorless cards that can be a huge help, regardless of who you've selected. The Byrdonis Egg, for instance, transforms into a colorless card with huge upside and seemingly no downside, other than having to wait for it, that is.