Nilshais had worm-like bodies with three clawed legs and three slender tentacles for arms that each ended in three stubby but strong fingers. It had a bulbous head that sat atop its worm-like body with three eye stalks that each ended in a large eye. It also had three membranous wings that tightly folded along its trunk when not required. A nilshai had a soft, slimy, and slick hide that was mottled blue and black, but their flesh underneath was extremely hard.
When it felt threatened, a nilshai profligately hurled spells at a foe in order to kill them as quickly as possible. When faced with spellcasters, a common tactic of the nilshai was to cast a spell and then immediately cast a counterspell to dispel any magic that might be forthcoming from its foe. Their hard bodies were difficult to damage without magical weapons.
Because of their trilateral symmetry and alien minds, nilshais were adept at multitasking. Able to do more at a time than others, they could attack or use their magic, and then use magic again immediately afterward. It also meant they had keen senses and were skilled at using unfamiliar magical items.
These malevolent alien beings dwelled on the Ethereal Plane, or on the demiplanes drifting deep within it. However, they occasionally entered the Material Plane on expeditions to seize lore, slaves, and magic. They desired to conquer new realms and planes for their own alien kind. The sought out and even paid highly for king's tears, on the chance that they might contain star elf lore.
When on the Material Plane, a nilshai was almost always pursuing a purpose, whether searching for magical items, engaging in espionage, or looking for slaves (intelligent humanoids) to abduct to its home plane. It accepted no interference in its activities from residents of the Material Plane. They had only contempt for life on the Material Plane.
In particular, they had invaded Sildëyuir, the extraplanar home of the star elves, for several centuries after stumbling across it and learning of its existence. Despite it not being coterminous, they bored several dark portals through the dimensions to access the demiplane, in order to invade and twist it to their own strange ends, by altering the plane's very essence. Each invasion was harder to beat back than the last, and by 1372 DR, great stretches of Sildëyuir had been corrupted. From these emerged monstrous non-living things that plagued the silver woods and assaulted the stars elves' citadels. The next great incursion seemed only just around the corner and they threatened to destroy the plane itself. Some star elves even called for their race to return to the Yuirwood and abandon their realm to the nilshais, while others wanted to seek allies instead.
Very few in the Material Plane or Sildëyuir knew anything about nilshai society or their activities. They seemed to be a part of the same extraplanar ecology as ethereal filchers and ethereal marauders.
Nilshais were encountered either operating alone or in infestations of up to four together with three to six ethereal marauders.
The cities of the nilshais on the Ethereal Plane were strongholds built of ethereal stone, bizarre in shape and with complex chambers and twisting passages that were enough to drive a trespasser mad. Large strongholds could have about five-hundred nilshai residents, but most nilshais lived in communities of about fifty. A stronghold was led by its most powerful sorcerer, who ruled with absolute authority and assumed full obedience from all the others.
Within their strongholds, nilshais kept ethereal marauders as pets and ethereal filchers as servants. They also had ethereal slayers. Nilshais sometimes used other, weirder, creatures as guards and raiders.
Around 1372 DR, the nilshai Tolg'byri discovered and opened a forgotten portal from the Yuirwood to the Night Realm, a cut-off fragment of Sildëyuir and site of the tomb of Mourel Duskwalker, an evil star elf mage of times past. Tolg'byri used the tomb as a base while amassing monstrous and mercenary forces for an attack on Sildëyuir and search for ancient star elf knowledge that could aid them. However, they disturbed Mourel's undead spirit and experiments, who fought a defense. Then Tolg'byri was betrayed, challenged, and slain by his lieutenant, Savera, and later raised as a zombie by Kyjal Stardancer.
In the Year of the Bent Blade, 1376 DR, nilshais were found gathering in great numbers in the Yuirwood, mostly around the ancient elven menhirs. They inflicted vicious attacks on the people of Aglarond, so the Simbul placed a bounty on nilshai hides.
- Richard Baker, Matt Forbeck, Sean K. Reynolds (May 2003). Unapproachable East. (Wizards of the Coast), pp. 67–68. ISBN 0-7869-2881-6.
- Richard Baker, James Jacobs, and Steve Winter (April 2005). Lords of Madness: The Book of Aberrations. (Wizards of the Coast), p. 14. ISBN 0-7869-3657-6.
- Richard Baker, James Jacobs, and Steve Winter (April 2005). Lords of Madness: The Book of Aberrations. (Wizards of the Coast), p. 8. ISBN 0-7869-3657-6.
- Richard Baker, Matt Forbeck, Sean K. Reynolds (May 2003). Unapproachable East. (Wizards of the Coast), pp. 106, 112. ISBN 0-7869-2881-6.
- Greg A. Vaughan (September 2006). The Twilight Tomb. (Wizards of the Coast), p. 17. ISBN 0-7869-3947-8.
- Richard Baker, Matt Forbeck, Sean K. Reynolds (May 2003). Unapproachable East. (Wizards of the Coast), p. 101. ISBN 0-7869-2881-6.
- Richard Baker, Matt Forbeck, Sean K. Reynolds (May 2003). Unapproachable East. (Wizards of the Coast), p. 94. ISBN 0-7869-2881-6.
- Richard Baker, James Jacobs, and Steve Winter (April 2005). Lords of Madness: The Book of Aberrations. (Wizards of the Coast), p. 177. ISBN 0-7869-3657-6.
- Greg A. Vaughan (September 2006). The Twilight Tomb. (Wizards of the Coast), p. 3. ISBN 0-7869-3947-8.
- Greg A. Vaughan (September 2006). The Twilight Tomb. (Wizards of the Coast), pp. 20–21. ISBN 0-7869-3947-8.
- Brian R. James, Ed Greenwood (September 2007). The Grand History of the Realms. Edited by Kim Mohan, Penny Williams. (Wizards of the Coast), p. 158. ISBN 978-0-7869-4731-7.