Melkor in unformed Arda, before First Age

When the Valar entered into Arda and began to shape the unformed matter, Melkor saw the Field of Arda and claimed it for his own. However the other Valar took Manwë to be their lord.

The aim of the Valar was to prepare the world for the coming of the Children of Ilúvatar, while Melkor had thought only for his own desires. Bitter, Melkor set himself against the other Valar. Whenever the Valar worked to better the world, Melkor disrupted their efforts.

For a long while, Melkor fought alone against the might of all the other Valar and Maiar of Arda, and he long held the upper hand. During this time, though Arda was kept effectively barren, it began to take an unintentional shape through the constant upheavals caused by the First War.

Then one day, the mighty Vala Tulkas descended to Arda, and his strength tipped the balance in favor of the Valar. Melkor fled before him, and left Arda for a time.