This phrase appears in a song Tolkien wrote, and what was the main theme of The Hobbit movie:
Far over the Misty Mountains cold
To dungeons deep and caverns old
We must away ere break of day
To find our long forgotten gold.
Tolkien was an outstanding linguist, and I guess this line is one of the antique structures he often used in the LOTR’s poems. To my knowledge, the line means “We must go away before dawn”, but omits the main verb completely. What is this exactly? Is this structure used nowadays?