The phrase "boss room" refers to a special area or environment in a video game where players face off against a powerful and challenging enemy called a boss. Full definition
Consequently, Fierce Deity Link, the deity that Link becomes when dawning this mask, has the ability to defeat any Boss in the game quite easily using the power of the Double Helix Sword and Sword Beams, but can only exist in boss rooms. (zeldadungeon.net)
Each room, including boss rooms, have «Tests of Faith,» modifiers that manipulate some part of gameplay, to make things easier, more challenging or funny. (darkstation.com)