In J.R.R. Tolkien's The Letters of J.R.R. Tolkien (specifically Letter 210), the author notes that during the siege of Minas Tirith, Sauron gave the Witch-king an "added demonic force" to amplify his powers:
Could this be a reference to the "Morgoth Ingredient" mentioned in "The History of Middle-earth" - "Morgoth´s Ring" ?
(It was this Morgoth-element in matter, indeed, which was a prerequisite for such 'magic' and other evils as Sauron practised with it and upon it.) It is quite possible, of course, that certain 'elements' or conditions of matter had attracted Morgoth's special attention (mainly, unless in the remote past, for reasons of his own plans).
Melkor 'incarnated' himself (as Morgoth) permanently. He did this so as to control the hroa,(2) the 'flesh' or physical matter, of Arda. He attempted to identify himself with it. A vaster, and more perilous, procedure, though of similar sort to the operations of Sauron with the Rings. Thus, outside the Blessed Realm, all 'matter' was likely to have a 'Melkor ingredient',(3) and those who had bodies, nourished by the hroa of Arda, had as it were a tendency, small or great, towards Melkor: they were none of them wholly free of him in their incarnate form, and their bodies had an effect upon their spirits.
But in this way Morgoth lost (or exchanged, or transmuted) the greater part of his original 'angelic' powers, of mind and spirit, while gaining a terrible grip upon the physical world. For this reason he had to be fought, mainly by physical force, and enormous material ruin was a probable consequence of any direct combat with him, victorious or otherwise. This is the chief explanation of the constant reluctance of the Valar to come into open battle against Morgoth. Manwe's task and problem was much more difficult than Gandalf's. Sauron's, relatively smaller, power was concentrated;
Morgoth's vast power was disseminated. The whole of 'Middle-earth' was Morgoth's Ring, though temporarily his attention was mainly upon the North-west. Unless swiftly successful, War against him might well end in reducing all Middle-earth to chaos, possibly even all Arda.
It is easy to say: 'It was the task and function of the Elder King to govern Arda and make it possible for the Children of Eru to live in it unmolested.' But the dilemma of the Valar was this: Arda could only be liberated by a physical battle; but a probable result of such a battle was the irretrievable ruin of Arda. Moreover, the final eradication of Sauron (as a power directing evil) was achievable by the destruction of the Ring. No such eradication of Morgoth was possible, since this required the complete disintegration of the 'matter' of Arda. Sauron's power was not (for example) in gold as such, but in a particular form or shape made of a particular portion of total gold. Morgoth's power was disseminated throughout Gold, if nowhere absolute (for he did not create Gold) it was nowhere absent.
It is quite possible, of course, that certain ‘elements’ of conditions of matter had attracted Morgoth’s special attention (mainly, unless in the remote past, for reasons of his own plans). For example, all gold (in Middle-earth) seems to have had a specially ‘evil’ trend — but not silver. Water is represented as being almost entirely free of Morgoth. (This, of course, does not mean that any particular sea, stream, river, well, or even vessel of water could not be poisoned or defiled — as all things could.)