The Silmarils

[Aredhel Idril Telcontar]

But Ungoliant had grown great, and he less by the power that had gone out of him, and she emeshed him in a web of clinging thongs to strangle him.

Though Melkor was the greatest (arguably) of the Ainur and the most powerful of the Valar, Morgoth, however, was just a pitiful Dark Lord, striving for dominion. I have always imagined the two as different entities in one body.
Yes, yes, 'Morgoth' is just a name that Feanor gave Melkor, but there seems to be a huge difference between the two, if you look at it in a certain way.

-Aredhel