Othrondil

What we know

Former homeland of the Dusk Elf people; also called the Forest of Twilight (Session 9 @ 01:48:15)
Ruled by a council of princes led by Erevan Lowenhart — master of bladesong; lion sigil and style (Session 9 @ 01:48:15)
Kasimir Velikov was a mage and scribe in Erevan's court there; ~100 years old when conquest began (Session 9 @ 01:48:15)
Dusk-elves arrived in the valley after the First Folk but before Strahd von Zarovich (Session 9 @ 01:46:29, 01:47:08)
King Barov II demanded dusk-elf fealty and tribute to the borders of old Zarovia; Erevan Lowenhart's refusal ignited war (Session 9 @ 01:48:15)
Rahadin betrayed his people and joined Barov's forces; shattered Erevan's blade and executed his family at Othrondil, ending the royal line (Session 9 @ 01:48:15)
Dusk-elves were subjugated; resistors hunted; after Barov's death a rebellion was quashed in days by Strahd von Zarovich (Session 9 @ 01:48:15)
Genocide left fewer than a hundred survivors, who were led by the Vistani to sanctuary in the valley that Strahd later named Barovia (Session 9 @ 01:48:15)
Party has not visited; knowledge comes only from Kasimir Velikov's account (Session 9 @ 01:48:15)

Named in Kasimir Velikov's historical recounting at the Vallaki Vistani Encampment (Session 9 @ 01:48:15)

Unconfirmed: whether Othrondil still exists or where it lies relative to Barovia (Session 9 @ 01:48:15)
Unconfirmed: whether monuments near Vallaki were dusk-elf work — Kasimir Velikov said no; they are First Folk remnants (Session 9 @ 01:46:29)