On the FR lore.
I honestly kind of tabbed out on the 4e to 5e transition.
Unther is back, so what happened to Tymanther then? Are the dragonborn back in Dragon-hell world?
Why are they are PHB race then?
Your really better off reading two novels by Erin M Evans that covers it. Ashes of the Tyrant and The Devil You Know, or even just read the entire Brimstone Angels series period, which starts in 4e, but going into the Sundering by the end, its one of the best Forgotten Realms series period, Erin M Evans.
Spoiler Alert, just in case you don't want to wait.
When the Sundering hits, the descendants of Unther (and Mulhurondi who were occupying it when the Spellplague hit), were transported to Toril just as they were about to invade a Genasi city on Abeir, which was apart of an Empire of Genasi called Shyr which was ruled by a elemental God like Primordial. See the Genasi had enslaved the Mulhorandi and Untherites, putting the Mulhorandi in charge of the Untherites. When Gilgeam is reborn on Abeir he leads the Untherites in a revolt against the Shyr Empire. The Sundering messes that plan up by transporting most of the Untherites back to Toril, including Gilgeam.
This is a good thing for Gilgeam as he steals credit for returning the Untherites to their homeland. Also Toril is alot more high magic then Abeir is so Gilgeam starts to grow alot in power, I mean alot.
He rebuilds Unthlass the old capital, partly with the help of captured Dragonborn refugees from the Chaos of the second Sundering which is changing the land or transporting Dragonborn back to Abier, but in a haphazd sort of way, not all at once.
And Gilgeam is not the only Untherite God involved. Enlil is trying to find away back to FR and he speaks in the head of Dragonborn who functional becomes his prophet. He makes a deal with the traditionally none religious Dragonborn, and he ends up saving both of their major cities from being transported by the Spellplague, so while Tymanther survives, it loses most of it's territory to Unther and maybe some to a restored Mulhorand, but saves a strip of land around it's two large, Fortress like cities. This of course leads to war Between Unther and Tymanther, which ends in a Stalemate basically.
Also Dragonborn refugees that fled from returned abeir across the the sea before the Sundering and Dragonborn who left Tynanther before the Sundering and their descendants are totally unaffected, so there is a huge Dragonborn diaspora already throughout the Forgotten Realms, so there are Dragonborn from different cultural traditions as well.
