Dean had suspected that the room of mirrors existed long before the tower. It was not something he could prove but something in the way that the mirrors changed and moved gave him the impression it was something he didn't understand at all. It was something of the God and this close to Castiel- here in the Tower- it was something he found difficult to concentrate on. The mirrors showed the world. They showed possible futures and futures that might have been. They showed the past as it was and how it would have been. They showed distant planets and the hopes and dreams of men.
Dean was drawn to them with a morbid fascination.
He knew if he reached out to touch the glass he would see his brother, happy in the history that Castiel had given him. He would see everything.
Castiel had shown him the mirrors soon after he had been accepted as consort. He brought him up the stairs, Castiel's hand was hot and large on the small of his back, the power of his Godhood rolling off him in waves thick enough to taste, with desire and love and passion and need and care. He had spoken of worry but when Castiel's power was so rich Dean found it hard to think, overwhelmed by the smell of him, the touch of him and his very divinity. It was the third party in their marriage.
It was like a sea that consumed him, no matter how Castiel stood, like Canute, ordering back the tide.
Castiel had led him up the staircase to the mirrors. The room had never before denied him, but unexplored, and Castiel spoke in his low voice. He said "These are windows to the world, these," he led him to the largest mirrors, "are my great dilemma," there were two of them, with heavily gilt frames and dark spots. "These are the two futures between which I must choose. Do I save mankind and damn the planet, cursing them to eternally wander the stars, or do I destroy mankind and all its wonders but save the planet."
Dean had looked at him, seen the worry on his face before he looked into the mirrors. The one on the left showed the future of mankind, eternally wandering the stars, building colonies on distant planets but there was a coldness there. In the other there was a great forest and birds singing. There were towers and skyscrapers here and there among the trees. These were the horrors and reasons, this was why the angels had released Lucifer, this was why Raphael had started his pogrom.
Dean thought about it for what might have been days. He sat Indian style on the floor, cross legged and eating if Castiel brought him food, and drinking if Castiel brought him water. He stayed because Castiel wanted his opinion, he felt so cherished in that knowledge, knowing as well it was the force of the Godhood. Then he shrugged, pulled down another mirror from the wall, thick as it was with them, aligned eight or nine high, and placed it in front of the two mirrors.
When Castiel came in Dean was polishing off his last sandwich. "There's your solution," he said, "not one, both!" And Castiel looked at the mirror propped there by others he had taken off the walls, at the very simplicity of it and laughed. It was not a mocking laugh, but one that realised that the very simplicity was what had eluded him and relief that the decision was made, and he took the sandwich from Dean's hand, smiling and laughing with him, and kissed him, and Dean thought he might die from the overwhelming love he felt at that moment, and wondered in the dark part of his mind that belonged only to him, immune to Castiel's divine nature, how much of it was his own.
Castiel used the mirrors to gather a conclave of gods that very day, lips still swollen from kissing Dean, hand still on the brand on his arm, and they set their plan in motion. They had met in Las Vegas, gods gathered and death sat among them like an equal to their number. Then the Gods scoured the earth, leaving alive only a small fraction of its population, destroyed its infrastructure and created natural disasters the like of which the world had never seen. And Dean lay in the god's great feather bed, drunk on love, and didn't care that he had done it, only that Castiel was his God and that Castiel loved him.