Sometime post TFA, the war between The Resistance and the First Order is put on hold and they are forced to form a temporary alliance when they learn that a far worse threat is imminent. The Reapers; an ancient, sentient machine race; are returning to wipe out the galaxy. Neither side can hope to defeat them alone.

If you're not familiar with the Reapers: http://masseffect.wikia.com/wiki/Reaper

I only really want the Reapers involved, not any of the other Mass Effect characters or alien races.

Any pairing(s) or none is fine.

How they find out about the Reapers is up in the air, as well. Perhaps Luke or Snoke.