Nine Layers of Sky — Liz Williams

A former Soviet scientist, Elena Irinovna cleans office buildings. Ilya Muromyets is an eight-hundred-year-old remnant of Russia's glorious past. Now he's a broken man. They are brought together by a strange artifact made of an unknown metal that offers a glimpse into another dimension, creating a dangerous breach in a world they both only thought they knew....

Now the woman of science and the man of myth will discover their destiny--and each other--in a place where dreams...and nightmares...don't just become reality. They are reality.

It's the only reality worth defending--even if it means destroying the world to succeed.