Valravens are special troops which appear in the Raid Boss and Invasion Events and will reward you with Sigils if you defeat them.
They will randomly appear in Raid Boss and Invasion battles, if you can defeat them before they fly away and if you win the battle, you will receive Sigils.
If you lose the battle the Valraven won't drop Sigils.
You can also receive the Valraven troop by winning Vault Battles, then you can add it to your Team.
Valravens on a player's team will NOT drop Sigils.