Off-Stage

Dominic Flandry's continuing villains, the Merseians, were introduced in the second Captain Flandry story, "Honorable Enemies," but were later retconned into:

the earlier Polesotechnic League period of the Technic History;

the Young Flandry Trilogy;

a revision of the first Captain Flandry story, "Tiger by the Tail."

In any given story, Merseians may be on-stage, off-stage or not involved. In the revised "Tiger by the Tail," the Scothanian barbarians had been armed by another race that might have been the Merseians trying to foment trouble for the Terran Empire. Inquiring further, Flandry discovers that:

"The Frithian kings had brought the nations of Scotha under themselves a century ago, and gone outward to the stars. Certain tales suggested their tutors had indeed been Merseians; however, no such beings had been seen for a long time."
-Poul Anderson, "Tiger by the Tail" IN Anderson, Captain Flandry: Defender Of The Terran Empire (Riverdale, NY, 2010), pp. 241-276 AT p. 253.

Here, the Merseians are the perfect behind-the-scenes villains, not seen, not described, not active here and now but nevertheless possibly responsible for the Scothani threat to the Empire.

In "The Game of Glory," Merseian gunrunners are expected to arrive on Nyanza. Their way has been prepared by their agent of a different species, A'u.

In "A Message in Secret," Flandry can see that Altai is being armed by some other power, probably Merseia, then he is told:

"'It is the Merseians, great greenskinned monsters with long tails, the Merseians, I tell you, who come in secret from a secret landing field. I have seen them myself, walking these halls after dark...'"
-Poul Anderson, "A Message in Secret" IN Captain Flandry..., IV, pp. 341-397 AT p. 356.

This is also effective. The Merseians remain off-stage but are described as monsters seen at night. It is as if his informant has seen demons conjured.

In "The Warriors from Nowhere," a human villain, Duke Alfred, plots to detach the Taurian Sector from the Terran Empire and we later learn that he would have taken it under the protection/possession of the Merseians. Again, a remote but effective off-stage threat.