The wiki should have it. But in a nutshell:
- Last War (any character from Khorvaire (the default area) would know and be affected by this unless from a very long lived race): The old king of the Galifar Empire died; afterwards, his children warred over who should become king/queen in violation of how it was previously done. This lasted almost 100 yrs, with every major nation (Aundair, Cyre, Breland, Thrane, and Karrnath) almost allying with every other nation and betraying them, ending only because of the Day of Mourning.
- Day of Mourning (anyone who has been in Khorvaire in the last 4 years knows about this): For years ago, Cyre was destroyed. One day, while armies were fighting, a magical storm overtook the entire country, stopping just at its borders. Anyone inside was killed. It is now a place of twisted magic and death where healing is muted, the dead continue to fight the same battles, and magic has taken on a life of its own literally.
Because no one has any idea who or what caused the Day of Mourning, everyone suspects it could happen again. Thus, the Last War ended because of the shock and fear of it. It still has political and economic repercussions (i.e., House Cannith is fractured because of it).
As for the Houses: You can be a member or sanctioned by a house if you are not of the Mark-bearing race, but only members of the Mark-bearing race can gain the mark (Except for PCs, who are expressly different. But it is considered very rare). So developing a mark could be dangerous either physically (they think you're aberrant and want to kill you) or politically (they are going to study you; involve you in devious plans, etc.). All ripe for story opportunities, but be aware of them. As an example, my half-orc artificer has the Cannith mark for various reasons; this has caused him to be hunted by Cannith (some want to kill him).
