Dusk Elf

What we know

Dark-complexioned elves with ashy skin; distinct from other elves party has seen (Session 9 @ 01:32:40)
Survivors of von Zarovich conquest; less than a hundred remain after genocide (Session 9 @ 01:48:15)
Once dwelled in Othrondil, the Forest of Twilight; ruled by council of princes (Session 9 @ 01:48:15)
Subjugated by King Barov II; rebellion quashed by Strahd von Zarovich (Session 9 @ 01:48:15)
Vistani took survivors into caravans and led them to sanctuary in Barovia valley (Session 9 @ 01:48:15)
Dwell in hovels south of Vallaki Vistani Encampment wagons; grim outcasts with clipped ears (Session 9 @ 01:36:11)
Baron Vargas Vallakovich bars dusk-elves from Vallaki as "malicious unhappiness" (Session 9 @ 02:03:34)
Dr. Rudolph van Richten suggested asking dusk-elves about old magic at butterfly standing stone near Lake Zarovich (Session 9 @ 00:47:17)

Kasimir Velikov — elder at Vallaki Vistani Encampment hovels (Session 9 @ 01:40:19)
Rahadin — traitor to the people; serves Strahd von Zarovich; also Ravenloft (Session 9 @ 01:48:15)
Erevan Lowenhart — historical prince; slain by Rahadin; ended royal line (Session 9 @ 01:48:15)

Hovel dwellers directed Sylalys to speak with Kasimir Velikov (Session 9 @ 01:37:41)
Kasimir Velikov recounted dusk-elf history to party in his hovel (Session 9 @ 01:48:15)

Unconfirmed: whether monuments near Vallaki were dusk-elf work—Kasimir said no (Session 9 @ 01:46:09)