The Great Barrier Forest (Dominaria Supplement)
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Quick summary below, expect revisions for flow and readability and [links].
The Druidian Brotherhood of ancient Ramosia created the Great Barrier Forest as a last ditch defense against the incursions of the Steppe tribes through the Dhazantar Gap. As the terrifying invasions of the Second Age reached a climax the shamen of the Eastern Brotherhood, the Rock Wardens, and the Riverwalkers called a grand summit of the Ramosi Druids just north of the King's Wood to discuss the portends and decide whether or not to break their neutrality and intervene on behalf of the Ramosi, Malkeiri, and Nurimanian Imperial Alliance. They decided that the great destruction wrought by the reaving steppe hordes from east of the Verge was upsetting the natural balance and Obad-Hai demanded action.
These three major tribes formed the Druidian Brotherhood, a temporary suspension of their usual enforced territories and responsibilities, in order to take the Druid Nations of the Ramosi Crescent to war for the first time since the [Time of Troubles?] in the First Age. The Brotherhood quickly rallied war parties and brought in allies from weaker tribes, while at the same time sending parties to the Imperial Alliance's war camps.
The druids fought in and among the Alliance for two years [link to section on war] before conditions on the front deteriorated to the point that something drastic had to be done. In short, the druids had the Alliance fall back while the Brotherhood moved much of their strength through the Horde's lines/territory in secret (animal forms, stealth, knowledge of the land, etc). Once established on either side of the massive Dhazantar Gap, the Brotherhood unleashed a huge ritual to bring a mighty forest into being in the Gap.
An old growth forest, imposing in size and stygian in its hulking depth, closed the Gap in a manner of days. Earthquakes were the local harbinger of its appearance and a terrifying exodus of local creatures brought evidence of the looming disaster further still. The fabric and borders of Magic itself trembled and grew sick as the Horde fled east; and people living in occupied land were forced to flee their homes and head west as the forest grew up. The Brotherhood fled just as fast, both terrified of the effect of their actions and falling ill from the profound rending of the natural world. The loci of the ritual died before they crossed the Daan River and the remaining Druids split by tribe and returned to their tribal lands. The Brotherhood would never reform, and Druids' reluctance to approach their creation for generations would have a profound impact on the nature of the place.