I love thinking about the domain of Irmo, who “loved twilight and flittering shadows”. The Master of Dreams and Visions plays but a small role in the stories but each line, especially the rich descriptions of his realm in The Book of Lost Tales, are so interesting to me.
I also love the potential for horror and the strange and uncanny within the powers and domain of each Vala and Irmo especially is fun.
His physical domain is a place of illusions and shifting paths. Glades with springs where the mere mist that rises off them is enough to lull the unsuspecting into a trance. Elves can become easily lost in those gardens and there are tales of souls who sleep for years before being found, entangled in scented vines and night flowers. Waterfalls reflecting kaleidoscopic colors and trees tended by strange pollinators.
Boundaries shift and blur in a thousand ways.
And of course the Maiar who once served him who now serve Melkor are capable of other terrifying deeds. The guardians of Angband’s nightmares, the crafters of cruel illusions and masks, who can deny sleep and induce it, who can wear a million faces or none at all.