You may perform the Heroic Actions Attack, Cast a Spell, or Rest while on the Boss Mat, following all normal rules.
If you move the Boss back to the Boss Mat, in order to attack the Boss you must move from the Lair Door Card to the Boss mat.
If all the heroes are on the Boss Mat, any Goblins in the dungeon will move toward the Boss Mat and can enter it.
Any attack the Boss (or any Goblin on the Boss Mat) uses while in the Lair only affects heroes on the Boss Mat, and vice versa (this is a big advantage to keeping the Boss in its lair!).
For example, when the Boss is in its Lair, an attack that damages all heroes () only affects heroes on the Boss Mat, and not heroes in the dungeon, including on the Lair Door Card.
While a Boss is in its Lair, heroes outside cannot use Missile or Spell Attacks against it. All your attacks must be done while on the Boss Mat. Nor can you Cast Spells from outside to affect any heroes currently on the Boss Mat.
If you are unconscious while on the Boss Mat at the beginning of your turn, you must Rest.