Daelkyr

The daelkyr are extraplanar creatures who appear as preternaturally beautiful humanoids that are native to the realm of Xoriat.
The daelkyr attempted a planar invasion during the time of the Dhakaani Empire, releasing hordes of their aberrant creations into the material plane. It took an alliance between the goblinoids of the Dhakaani Empire and the orc druids of The Gatekeepers to repel the forces of the daelkyr, cutting off the plane of Xoriat from Eberron and sealing the remaining daelkyr and their servants within Khyber.
At least six daelkyr remain on Eberron, though they have yet to make a unified attempt to free themselves from Khyber or to restore the connection between Eberron and their home plane. Immortal, the daelkyr are endlessly patient and, as the lords of the realm of madness, their goals and motivations are unfathomable by mortals. They are responsible for most of the aberrations in Eberron, including beholders, dolgaunts, some symbiotic creatures, and many other monsters.
The daelkyr are perceived as a perfect paragon of the same race as the person viewing them (i.e., they may look human to humans, but would look like a bugbear to goblinoids). Scholars best guess is their true form may not be interpretable by a rational mind and the humanoid form is the nearest approximation. This is distinct to shapechanging abilities like those possessed by Changelings, as the Daelkyr is not consciously changing it's form, everyone who sees them sees something different.
According to books found in the Citadel of the Closed Circle, there are 6 daelkyr known to be active on Eberron. Each of them is imprisoned in their own demiplane that they cannot leave:

• Dyrrn, the Corruptor . Dyrrn's lair touches the Eldeen Reaches and Mror Holds.
• Belashyrra, the Lord of Eyes. Belashyrra's lair connects to Xen'drik and the Shadow Marches.
• Kyrzin, the Prince of Slime. Kyrzin's lair is beneath the Shadow Marches.
• Orlassk, the Lord of Stone. Orlassk's lair is a giant gargoyle that moves throughout Khyber, it is not found in any one place.
• Avassh, the Twister of Roots. It is not known where it's lair is located in Khyber, but it has spread it's roots to all of the woods and bogs of the world.
• Valaara, the Crawling Queen resides in The Deepest Hive. It is unknown where this is located in Khyber, and it seems that her minions dig tunnels all over the world. The author believes she has been active in Sarlona.

The daelkyr are also credited with the creation of many aberrations and monstrosities that can still be found on Eberron today. The following information was pieced together from several books in the library:

• Dyrrn created Dolgrims from goblins. They reproduce via mitosis, wherein a single Dolgrim splits into two dolgrimlings which then grow into full dolgrims over time.
• Dyrrn created Dolgaunts from hobgoblins. They reproduce by implanting their cillia into a hobgoblin's eyes, an intensely painful experience that leaves the hobgoblin in agony for days as it slowly transforms into a dolgaunt.
• Dyrrn created Dolgarr's from bugbears. They look someone turned an ape inside out, and they seem incapable of reproduction on their own.
• Dyrrn created Illithid's from something called Gith, the author seems to think these are creatures from another world?
• Dyrrn created Neogi from Dwarves, twisting their forms into giant insect-like creatures and amplifying the dwarven greed into something uncontrollable.
• Belashyrra created Beholders and Beholder-kin, and the author notes that whilst most of these are malicious and arrogant, some seem to be quite cordial. There is an account of an individual having a philosophical debate with one.
• Kyrzin supposedly created mimics, and all oozes come from the Prince of Slime originally.
• Kyrzin is also credited with creation of something called a "Gibbering Mouther". No further information on that is provided.
• Orlassk is credited with the creation of basilisks, cockatrices, medusas and gargoyles, though the book also notes that the medusae deny this, and claim to have been created by The Shadow.
• The Twister of Roots created shambling mounds and myconids, and is possibly responsible for the creatures called "blights"
• Valaara doesn't use the minions of the other daelkyr, and hasn't created particular new monstrosities. Instead, it seems she transforms people into insects.