A couple notes:
IMLAD MORGUL (S.: ‘valley of black magic’) Valley in the western side of the Ephel Dúath, through which the Morgulduin flowed.
Minas Morgul was located at the head of the valley. During the WR the Captains of the West set fire to the
evil meadows of Imlad Morgul and broke the bridge across the Morgulduin, but even after the War the terror of the valley was too great for it to be resettled.
Also called Morgul Vale, Morgulvale, the Valley of the Wraiths, and the valley of Living Death. (II 388, 397 ff.; ’93 ed. 361, 369 ff.; ’01 ed. 345, 353 ff. III 197–98, 305; ’93 ed. 178, 276; ’01 ed. 167, 267. S 361. B 452.)
Foster, Robert. The Complete Guide to
Middle-earth:
Tolkien's World in
The Lord of the Rings and Beyond (p. 268). Random House Worlds. Kindle Edition.
Imlad Morgul ‘Vale of Sorcery’ (Sind.) – One of the western
valleys of the
Mountains of Shadow, lying only twenty miles north-east of the
bridges of
Osgiliath. It was guarded by the evil tower of Minas Morgul and watered by the Morgulduin, once the Ithilduin, flowing gently down through
Ithilien to the
Anduin. The valley, the tower and the river were all renamed in
Gondor after the fall of
Minas Ithil to the
Nazgûl (in 2002 Third Age). The waters of the Morgulduin were poisonous, and the loathsome white flowers in the meadows of the valley produced noxious vapours; rottenness and decay filled Imlad Morgul. At the end of
the Third Age the vegetation there was burned to the rock and the evil tower was thrown down for ever.
Tyler, J. E. A.. The Complete Tolkien Companion (pp. 412-413). St. Martin's Publishing Group. Kindle Edition.
Conclusion so far: a reference to more than just teh
Nazgul.
the Lord of the Nazgul is also known as the Morgul-lord, and other fascinating
titles. Morgul Vale seems to be a hang-out for all sorts of wraiths, poisonous flowers, a foul river, and much other nasty-ness able to twist the minds of
men and instill unnatural fear etc.