First mentioned in times long past (2013), Shin Megami Tensei X Fire Emblem is at last a real thing. Nintendo and Atlus recently showed off the planned crossover, which now bears the title of Genei Ibunroku #FE, or Illusory Revelations Sharp FE.
The setting tilts more towards the Shin Megami Tensei side of things, taking place in a modern Japan that exists alongside a hidden fantasy universe, from which the Fire Emblem characters emerge. How exactly this mixture works has not yet been detailed, though a trailer has been released on yonder YouTube, and the official site (in Japanese) is available hereabouts.
The title is planned for a winter release in Japan; a global release has yet to be announced.