The Crooked Letter (Books Of The Cataclysm Book 1) by Sean Williams
Twin brothers Seth and Hadrian meet a girl named Ellis while backpacking through Europe on holiday. She plays them off against each other; they, already having major rivalry-issues, have a huge fight. They all get on the train where a mysterious Swede attacks them all and stabs Seth to death.
Hadrian wakes in a sinister hospital to find the world transformed: every city in the world has “amalgrammed” into one huge city which is where he is, and then all the human inhabitants have been slaughtered to pave the way for the invasion of the mindless hungry god Yod. Hadrian sneaks out of the hospital and spends several chapters running through the city, falling in and out of the hands of various groups of supernaturals who hope to manipulate the situation various ways.
It seems that Hadrian and Seth are “mirror twins” who can cause the Cataclysm of the everyday world of the First Realm to merge with the quasi-afterlife world of the Second Realm so long as one live twin remains in the First Realm and a dead one is sent to the Second Realm: their need to be together will drag the realms together. This was all engineered by Yod.
Alternating chapters carry on with Seth in the even more bizarre Second Realm where he makes several friends who quest with him to the borderline into the Third Realm where they plan to ask the Three Sisters of Fate what to do. The Sisters control the timelines that branch out from everyone’s choices in life.
The book starts off somewhat slow but really picks up when Seth arrives in the Second Realm and all the mythology and Realm stuff begins to come clear. A wildly strange novel, it is filled with quasi-religious world folklore and delirious imagery.