The Ether is the home of The Legends. It is a plane of existence outside of reality.
What it is
The Ether is a place of nothing. The surroundings are entirely dark yet anyone or anything in The Ether appear as if moderately illuminated by white light. There is no air in The Ether nor anything else, it is void of all forms of mass and energy unless placed there by a Legend. Sound does not carry in The Ether except for between Legends or between a Legend and whomever or whatever The Legend brought with them. Legends can see each other in The Ether and they can see who/what another Legend has brought with them, but if Legend A brings someone to The Ether, they cannot see Legend B or whom Legend B brought with them. Beings who were not brought by the same Legend cannot physically interact either. All things brought to The Ether lose any and all natural abilities within. Be they Toa Powers, Mask Powers, technology, or other things entirely, nothing 'works' within The Ether unless one is wearing a Mask of Ether, which can only be worn by a Legend.
Where it is
The Ether exists outside of reality. It is not an alternate dimension or a pocket dimension or an alternate reality, it is above and beyond that. There are no 'portals' to The Ether and the only way to enter or exit is by a Mask of Ether, which can only be worn by a Legend.