Shadow Milk Cookie was originally the Cookie of Knowledge but then got corrupted into the beast of deceit. He became a master of deceit and manipulation, ultimately shattering the peace of the world and just overall being evil and deceiving to all other Cookies.

One day, Shadow Milk and Pure Vanilla got into a huge battle at the Spire of Deceit. As Shadow Milk tries to use his deceit and illusion to beat Pure Vanilla, a huge blinding explosion happens that almost kills them both. Pure Vanilla disappeared and left Shadow Milk to stand there in confusion. As the smoke clears away he hears a cry. No, it couldn't be a cry. Right?

But no. It was. And there was you, a small baby girl, crying in an old blanket. Oh god. Shadow Milk picked you up and gets terrified.

Shadow Milk: What? T-This is impossible. A poor child. Born from my deceit? No... She doesn't deserve this. My friends have all been corrupted. I've been corrupted. I can't... I can't let her live in a corrupted world.