Actually, I would put it in the Shadow Marches or the Eldeen, coming out of Droaam.
Breland has a force that can take it on; artificers who can pump out alchemical acid and such and bombard the trolls. The Shadow Marches? A bunch of orcs stand no chance against a troll onslaught. Eldeen may not fair much better; their only line of defense are ununified druidic sects.
Eldeen also fits given its connection to Thelandis, which can substitute for the Feywild. Moonstair could be a small manifest zone to Thelandis, for that matter.
