Dalketh is the name given to a group of races invading Eurthasia from somewhere outside of the Prime Plane of existence. Dalketh are utterly alien to the Prime Plane and anyone viewing them will be struck by how obvious this is, not just because the Dalketh are hideous in appearance (though they typically are perceived as such), but because their physiology and behavior is simply unlike any denizens of the Prime Plane.
The Dalketh invasion appears to have begun deep below the earth and most surface dwellers are unaware of the invasion, let alone the size and scope of it. Not a great deal is known about the Dalketh, though what is comes primarily from those who have opposed them in the Underdark such as the Delvarin. What is known to the wise is that the Dalketh are utterly evil by the standards of most sentient species. Dalketh corrupt or destroy anything that stands in the way of their expansion. What they corrupt becomes Dalketh, be it land, people or goods. Many Dalketh also prey upon the races of Eurthasia.
The Dalketh races seem largely to be hive oriented. Some have speculated that the Dalketh represent all the races of a plane that has been completely and utterly dominated by a single mind, so that every being from that plane serves a common goal. This theory has been contradicted by reports of Dalketh destroying other Dalketh. Still the unity with which the Dalketh typically behave goes beyond what most Eurthasian creatures are capable of.
If anything good can be said about the Dalketh, it is that they seem to have no interest in forging alliances with anyone in Eurthasia and thus have made enemies of every being they have encountered. Sadly, this does not seem to have impeded them in the slightest.