Tetra Element

Tetra Element is an item that can control which evolution Super Eevee will be in.

It was created by Professor Oak, who managed to create the device after thoroughly studying Eevee's DNA structure. Without it, Eevee would just randomly switch between evolutions, while the Tetra Element allows complete control over which evolution is expressed.

Use
The Tetra Element is a key item, and can be used via the items menu, quick-select and inside battle. When used you can select which form for Eevee to evolve into. Each piece of the tetra element will allow Eevee to evolve into a different form.

If you have more than 4 forms, you can press the cancel/menu button to switch selections.

There is no consequence nor limits to using the Tetra Element. Expect in battle, where using it costs a turn. (Although, you can use the item in any Pokémon's turn.

The Tetra Element is a key item and thus cannot be sold.

Location
You obtain the Tetra Element when you obtain Eevee, and thus is an unmissable item. It will initially come in three pieces; Normal, Water and Thunder. Additional pieces are unlocked as you progress through the game.

The number of Tetra Element pieces will carry over in New Game +