I've been working on this one for a while but I don't know how to finish it. So here's what I have so far.

A bit of thought process for those interested:

A lot of thinking went behind this one and a bit of backstory is needed in order to understand my approach for this character. 
Originally known as Melkor, he was one of the Ainur, the first beings created by Ilúvatar (God) before the beginning of Eä, the Universe. Melkor was the mightiest of all for he was blessed with part of the gifts of each of his twelve siblings. Being so powerful made him lust for even more power and he became jealous of his own creator for he had the Flame Imperishable, the power to create life. He sought it in the Void but the Flame was only with Ilúvatar and as a result Melkor turned against him and the Ainur, and sabotaged everything they ever created.
So when Melkor descended upon Arda, the World, he was forced to take on a physical form as it was a material world. First he was beautiful and fair, and was able to decieve the Elves, the First Children of Ilúvatar. Even the other Ainur would be blinded by the beauty of his many forms, but through the ages and the countless wars he took the appearance of a tall dark lord. With his powers diminished, Melkor was stuck in this last and final shape forever and was therefore named the "Black Foe" by the Elves, or Morgoth in their tongue. In this form, he suffered many wounds before he was cast into the Void, leaving Sauron, his greatest lieutenant, to carry on his vile deeds through the Second and Third Age.
So for this concept, I decided to overlay chronologically the many shapes he must have gone through since his creation. 
What we know about Morgoth from Tolkien's writings:
-He is a "god" or "fallen angel".-He likes chaos, pain, war, violence, iron, fire, ice, wastelands and Darkness.-It is assumed he is very tall.-He is the most terrible and hideous being of Arda, which means even more so than the Balrogs, Orcs, Trolls and Dragons that he himself created.-He has scars, so that means he still has flesh.-He has "acid" blood.-He wears a black armour and helmet but people can somehow see his facial expressions.-He wears an iron crown with the Silmarils on it.-He wields a huge black warhammer (but he is also seen with a mace, a sword and a spear).
So here was my thought process:
Firstly, he was a devine and primeval being. We know once he took on a physical form he was first considered extremely fair looking by the other Ainur, which probably means he looked very much like them, godly and beautiful. So I had to start with something we all identify as being godly and beautiful and I instantly thought of Greek mythology. I started with one of my favorite designs in human history, the Corinthian helmet. It can be seen on countless sculptures and paintings of Ares, Athena and other gods and heros of Antiquity.
Secondly, we know he took on the appearance of a tall dark lord. Tolkien describes him as hideous and extremely scary. We know he once took the shape of a huge mountain, so as a "god" I don't think that he just changed his wardrobe, I rather like the idea that he morphed into his next appearance and that's why I decided to add the more medieval aspect of the iron crown directly on top of the Corinthian helmet. But we have to somehow see his eyes and mouth so I thought once he got stuck into his last form, his helmet and his rotten flesh merged over the ages to a point where it is almost impossible to differenciate them. So his armour is now basically him and that's why his helmet has a mouth. I also like the idea that his crown grew out of his skull, like Lucifer's horns, and keeps on growing with every century that passes. I also think the Silmarils need to look like they're stranded on the crown, being forced by Morgoth to stay in place. So he didn't just attach them, he welded them straight into the iron spikes.
Lastly, since I think he really needs to be disturbingly unsettling, I thought a "horror" touch was unavoidable. We know he still has some flesh and blood (probably almost black like Orcs), so I wanted to show a bit of that through the iron parts. He obviously likes pain and we know he has scars so I've always imagined him a bit like the Cenobites from Hellraiser who inflict torture on themselves. Jason Voorheers' mask in the later films of the series is also an inspiration. 
That's about it! It might not be my final concept since I'd like to explore other ideas keeping the same philososphy behind it. Maybe I'll try a full body version and play more with the chaos aspect of his character. We'll see!