A type of shadow temple similar to the twilight temple, link enters a roundish like boss room. he walks around then looks up. a dark substance hangs from the ceiling it suddenly drops. link quickly rolls out of the way. the substance slowly starts to rise then without warning lunges itself at link. link puts up his shield to defend himself but the dark substance seems to pass through him entirely. link looks infront of him then left to right but sees nothing. He turns around to see a mirror image of himself, but its...DARK. the substance seems to have adopted links appearance!

DARK link sort of like the one in SSBB (and the mini boss in the water temple in OoT but much more defined and powerfull) he will have all your weapons and all your weaknesses. though dark link can hover and run a bit faster than you, and his attacks are twice as powerfull.

there could be a number of ways to defeat your Evil twin. take him in combat (this would be hard and take a while)

when hitting him he will fall back and his chest will be open . hurry and shoot some light arrows at him! (if theres even any light arrows in the next zelda game)

you could distract dark link while whoever your companion is in the game ( perhaps the master sword lady) could do.. well ..whatever they do to attack.

there could also be sunlight or something that you can activate in the room then use maybe perhaps the mirror shield similar to the Ikana castle boss battle, to direct light at the DARK link .

depending on how far you are in the game will determine the difficulty of the boss

just an idea.