As mentioned, guisels are the only natural enemy of the Shroudlings in the Mirror Realm. They are an alien life form, native to that peculiar plane of existence. Possessing of an intelligence that is not quite sentient, but superior to that of an animal, the guisel is dangerous enough without being intelligent as a humanoid.
Guisels appear similar to a snake or an eel, approximately ten to 12 feet in length. A pair of vestigial wings are located about two to three feet behind the head/mouth giving the guisel the ability to raise it's head, and even rear up for attack. The rest of the body is supported by slender but deceptively strong insectile-like legs with two small claws at the tips. The head of the guisel is mostly a large angled mouth, with multiple rows of sharp teeth through which drips a deadly corrosive green acid substance. They are eyeless, the top of their heads being covered with rapidly beating cilia. They are usually a single color with two stripes running down their length that are a different color from the rest of the body and each other.
Their virtues are unhappily many. They are deceptively strong. The bite of a guisel can snap an ordinary or lesser enchanted blade like a wooden dowel. They are damn near indestructible, most weapons of even superior caliber being useless against them. The most heinous of wounds will not deter them, as even the tip of their tail will continue the fight sans the rest of the beast. Given the opportunity they regenerate. Spells are all but useless against them, serving to only slow them down a few seconds depending on the power of the spell, while the guisel pauses to literally 'eat' the energy of the magic (they seem to excrete refrigeration). Magical powers supported by construct based power sources, such as spikards, appear to slow them down long enough for the wielder to consider his/her options, but only offer temporary help.
A guisel also seems to have an innate ability to track someone throughout the Mirror Realm, even into Shadow which they can access through a mirror. They are hard to fool, but can be outrun, though a 'No-Field' will cause them to lose their spoor. The 'guise' are not hindered in the least by being outside of the Mirror Realm, but will seldom leave it unless there is a pressing reason. They can batter down a locked mirror given the time (and the relative skill of the shroudling who locked it).
In conclusion, the average guisel can be summed up as an unthinking, unfeeling, relentless, indestructible, killing machine. Legend suggests that no guisel has been killed by sword or spell alone. As Rhanda suggests in Roger Zelazny's short story 'The Shroudling and the Guisel', "Baffle it, imprison it, banish it. That might be better than trying to kill it."
Fortunately for us, guisels are extremely rare, and solitary hunters, not usually prone to associating too closely with their own kind.
Guisel's in Tony Pi's Paradox game play a more integral part than just challenging monsters to fight. While they appear much like Zelazny's, Tony has applied his own myth and magic to them. For example, in Paradox, the Guise have their own special antipathy with the bodiless Ty'iga.
Before the defeat of the Serpent of Dreams, and the formal founding and 'Golden Age' of Ambrium, the children of the Unicorn and Serpent fought against an organized force of the Guise. This was called the Vorpal War.
This unprecedented organization of guisel was attributed to a being known as the Guisel Queen. The Queen being fully sentient and intelligent acted as the leader orchestrator of her children. A comment is attributed to Darthene before the resolution of the Game which sheds some light on the relationship between the Guise and their Queen. "I believe he (Corwin) also has his own primal ally...the Ty'iga, the Guisel's natural enemy. Guisels are mindless bodies, ty'iga are bodiless minds..."
The Queen was beheaded with a fabulous artifact known as the Vorpal Sword. Her slayer was one of Lir's three shroudling sons, Rorschach (Madoc and Garat being the other two). Rorschach was lost and believed dead. The Vorpal Sword was somehow returned to Wonderland, a curious world that is both Reflection and Shadow.
The Queen's body is believed to be completely immortal, her head being the only vulnerable part of her body. Rorschach's beheading is believed to have only sent the body into hibernation, and legend suggests that the head was enchanted and hidden by Rorschach. The Guisel Queen's body left was left by the Dark Mirror.
During the millennial interregnum that preceded the Thousand Years of Slumber, Corwin and Darthene began their private war with the Board. The Great Game was interrupted by Darthene's imprisonment behind the Dark Mirror. Darthene, imprisoned, but still quite active tied her power to the Queen Guise, and through use of her magic and sheer force of will took control of the guisels. By either escorting or abducting Lorelei's daughter Meredith to the body, Meredith was able to employ the Serpent's Venom within her body to transform herself in the Queen Guisel's new head, thus resurrecting the Queen and becoming a living goddess.
Jacob received the Board from Oberon, and then parlayed with Darthene in a successful attempt to achieve stalemate in the Game. While this was happening Thalion and his uncle Garat had found the Vorpal Sword and were directed by the Tir's Guardians (Pierre and Orson marshaling behind Corwin) to slay the Guisel Queen for the second time. Meredith separated from the Guisel Queen and was transformed into the Phoenix of the Abyss. The current whereabouts of the Vorpal Sword is unknown, as is the location of the body of the Queen of the Guise.
Rorschach has only very recently been revealed as alive.
Meredith's transformation is a critical event in the Paradox Campaign, and many subsequent events can be tied back to it.