This phrase appears in a song of the dwarves in Tolkien's The Hobbit:
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 such poems. To my knowledge, the line means “We must go away before dawn”, but omits the main verb completely. What is this structure exactly? Does it appear in contemporary English in any way?