The Night Circus by Erin Morganstern 

This is an urban low fantasy.  There are no elves, no trolls, no orcs, no hobbits.  But there is magic and illusion.  Two very old magicians have an ongoing game they play with other people’s lives.  Each finds a special child with inherent abilities.  They adopt the child and train them.  Once the bet is placed, each child receives a ring, which burns itself into their finger.  Neither will be introduced to the other.  In this case, the venue becomes a circus which opens at night.  Those people who are involved with the circus find that they have become virtually frozen at the age they were when they first agreed to conceive and build the circus.  But they are not immortal.  One of the contestants enhances his own looks to make himself seem ordinary and serves as the assistant to the circus proprietor, who is not one of the magicians.  The other, a superb illusionist has her own act in one tent of the circus.  Each think that it is their work that sustains the circus.  Little by little they recognize that the other contestant is making changes and additions to the circus.  For years, this is how they compete.  Eventually they find each other and go beyond mere introduction to becoming lovers.  The problem is that there is only one way for a contestant to win.  The other must die.