The Last Watcher

The tower was impossibly large and impossibly black. It rose high in the green sky. 

Aaron has watched over the tower his entire adult life. He is now an old man, and climbing the steps to his throne, his water seat, is not as easy as it once was. He has chest pains and is sometimes short of breath. His life was coming to an end.

The creatures in the tower know this. They have waited patiently. Their chance at freedom is coming. 
The tower is cracking. The Old Ones are stirring. 

Another watcher must be found. 

Dean loves bodybuilding and fencing. On the day when Sparrows go mad, he enters a unisex bathroom and exits into a barren, dust-riddled world, with a green sky and nothing for miles around--except a staircase. 

The old Watcher meets the new Watcher. 
Not all Watchers are destined to duty when fate steps in.