lvl 2 player at the lvl 15 end-game citadel: I attempt to break down the walls of the impenetrable fortress with the Sword of Doom

"A bright flash emanates from the place of contact as the two unbreakable items collide. It expands into a small explosion in an instant, letting out a deafening noise. When your eyes clear from the flash, ears still ringing, a small, 2 foot deep crater is broken out of the 5 foot thick unbreakable wall. Half the sword is vaporized from the heat of the impact, and the powerful runes are destroyed. The shockwave hits your character in a rush of heat and force. Roll a Dex save (or a little force damage).

Many people far and wide heard the impact. A dragon is rising in the distance from the tower of the dark mages... Wanna run?"