There is a quest series in Greenshade involving the undead Blackroot Clan, who originally fought in the Bosmer tribal wars under the leadership of General Malgoth. I'll link to the text of the quest to jog your memory if it's been a while: https://elderscrollsonline.info/quests/right-of-theft

This tribe is described as warlike and bloodthirsty, but by all indications, they seemed honorable. General Malgoth's sense of honor is strong enough to bring him back from death to respect the Right of Theft. During the quest, you find a note from General Malgoth that indicates he wanted to unite the Bosmer people so they would be better able to defend the Valenwood from their common enemies. It also says that he chose to attack Vullain because the Bosmer there had forsaken Y'ffre in favor of the Nereids.

Knowing they would lose the battle against the Blackroot clan, the villagers of Vullain asked the Nereids for help, who suggested they poison themselves. This way when the Blackroots observed the Meat Mandate, they would be poisoned also, which is how both clans met their end.

At the end of the quest, you have the choice to free General Malgoth and the Blackroots from undeath, allowing them peaceful rest, or to command them to fight against the enemies of the Dominion. I chose to free him and he said that the Ooze is the only place his tribe belongs. It seemed like they were somehow condemned to the Ooze after losing the battle of Vullain. I dont understand how this could have happened. The Blackroot clan seemed to be following traditions and defending the Green. Why was the Blackroot clan condemned to the Ooze?

Did the Bosmer of Vullain, who we know had forsaken Y'ffre, poison themselves with some plant based toxin? If you eat someone that ate a plant... does that count? xD That seems to be the reasonable conclusion but it's not indicated anywhere in the quest text or note text. I feel like this is really minor and pedantic but if there's more information about them I'd be interested to hear it.