Master Emerald (InstanceTopic, 8)

From Compile Worlds

The Master Emerald is a key item in the Sonic series. It first appears in Sonic 2 Beta, but was retconned as a mere pipe blocker (though that's one expensive-looking pipe blocker if I may say so myself). More importantly, it appears in Sonic 3 & Knuckles, Sonic Adventure and Sonic Adventure 2, where Knuckles supposedly guarded it. It was capable of controlling the power of all the Chaos Emeralds, kept Angel Island afloat, and sealed away Chaos and Tikal.

After that, though, it becomes a generic Mac Guffin used for the most obscure purposes or ignored altogether, as Tikal died and Chaos reappears wherever he pleases just because the not-so-badass-any-more Eggman made a few insignificant threats.