As the title suggests these are stand alone books that feature in their own worlds. Morningstar is a new take on the Robin Hood myth with Vampyres mentioned and featured. Although this novel could be attributed to the Drenai books as there are mentions of Agnostins and Ventria in the novel.
Dark Moon sees the struggle against the Daroth an evil warlike race of birdmen. With the people who vanquished them originally (the Eldarin) having vanished. It is also in this novel that we see messages pointing out that man is at his most inventive when in war. (A theme echoed throughout most of his novels)
Echoes of the Great Song foresees the Avatars a race who have lived as "vampires" (sucking the life out of the populace using crystals) face annihilation when a similar race escape their own destruction by transporting their continent to the Avatars world using conquest to take it by force, to which the Avatars unite with the very people who they had in subservience for centuries.
Knights of Dark Renown has a similar theme to Dark Moon with the world being split into Duchies. Here the knights who protected it have vanished through a door in time and space leaving one of their number who fled in terror. Then a new order of knights arrive bringing terror and fear to the population rounding up what is termed as lesser races to be sent to a new land.
The character lists may include some spoilers so please be aware of this.