There's not a ton to go on, but most of what we know about the Raen is that they've largely abandoned nomadic life and have settled in and around the areas surrounding Doma. Doma, as well as the rest of Othard, has been under Garlean rule for 25 years. However, recently Doma rebelled against Garlemald and the empire razed the city-state to set an example for the rest of Othard. This is the lore reasoning for the recent influx of Doman (and Raen) refugees coming to Eorzea.
Here's a few lore links you may find helpful!
-Race Lore
-Under Imperial Rule Lore
-Doman Religion Theories
Here's a few lore links you may find helpful!
-Race Lore
-Under Imperial Rule Lore
-Doman Religion Theories