The Green Lady died while traveling on behalf of the Aldmeri Dominion.
Finoriell died while on the shores of Pyandonea. She sought revenge against the Maormer after the death of her dear Silvenar, but fell while fighting them. Her soul went to rest at Treehenge, a sacred grove where all Green Lady's souls go after death.
She will find The Silvenar dead with the Vestige. When the Vestige gives her a sample of the poison, she will follow the trail with the Vestige. She will want to kill Ulondil, and the Vestige has the option to let her kill Ulondil or let him live. The latter will allow him to be interrogated.