The Aetherium Crest is a quest item found in The Elder Scrolls V: Dawnguard.
Quest
Lost to the Ages
The aetherium crest can be made at the Ruins of Bthalft by combining all four aetherium shards. It is the key to accessing the Aetherium Forge deep below the ruins, and is the primary building material of the Aetherial artifacts: the Aetherial Shield, the Aetherial Crown, and the Aetherial Staff.