Rahadin

What we know

Dusk-elf of Dusk Elf people; serves Strahd von Zarovich as agent (Session 9 @ 01:52:01)
Per Kasimir Velikov: cousin and traitor who slew hundreds of dusk-elves; clipped survivors' ears (Session 9 @ 01:48:15)
Party believes elven proclamation messenger at Barovia Village may have been Rahadin (Session 9 @ 01:53:22)
Messenger: ash-gray horse, twelve undead escort; foreboding presence within ~50 feet (Session 9 @ 01:53:22)
Sildir heard psychic screams of agony near messenger (Session 9 @ 01:54:35)
Kasimir Velikov: flee if facing Rahadin's wrath—do not fight (Session 9 @ 01:55:11)
Delivered Strahd von Zarovich's proclamation ending village siege; named Ismark Kolyanovich burgomaster (Session 4 @ 00:40:45)

Unnamed elven messenger at Barovia Village square during Session 4; identity as Rahadin discussed Session 9 (Session 4 @ 00:40:45).