Nintendo revealed to Eurogamer that you will need up to four Metroid themed amiibo in order to access everything that Metroid: Samus Returns has to offer. Two of these amiibo are already out (Samus and Zero Suit Samus), while another two (Samus and a Metroid) will be out for this game.

Nintendo confirmed that each of these will provide additional content for the game.

The new Samus Aran amiibo will unlock an extra energy tank to use along with Metroid 2 art within the game's art gallery. The big one here is the soft Metroid amiibo. This will reveal the location of nearby Metroid on your map. This amiibo will also grant you access to a new difficulty mode, Fusion, upon completion of the game. You will also get to wear the suit from the Metroid Fusion game.

The old Samus amiibo unlocks a missile tank and concept art. Zero Suit Samus unlocks another energy tank and the game's soundtrack.

Most of the unlocks don't seem too egregious, but locking another whole difficulty seems to be a bit beyond the pale.