Invasions

In world, are there early signs of the type of invasion? What do intelligent Invaders remember? Sentient creatures that arrive at the plane via an invasion remember relationships but not events.
They might remember their hometown only so far as they remember its precious to them but not why or any of the details.
Two orcs might remember that they hated each other but don't remember any of the events or things that led them to that hatred. Clerics remember the relationship they had with their old gods but not their name or anything not directly related to that relationship. That's why the names of the old gods are so precious and rare: to remember them, the person would need to have a relationship to the name directly. Not impossible, but very uncommon.
Introducing New Races This can be done through Invasion although you can have it be that the race has been around for awhile and their population is enough that they have enclaves in the major Strongholds and villages but not enough to form political blocks so people will know what they are but they will still be rare