80's SF novel, not by a big name.
Set in a feudal interstellar empire of perhaps a hundred worlds. From the point of view of an aristocratic fleet captain who falls in love with his commoner mistress and starts to question the social system. Simultaneously a space cloud/amoeba appears at the fringes of the empire and goes from a rumour to a nuisance to a mortal threat as it begins moving inward, enveloping planets and consuming all life on them.
Our hero falls foul of the emperor and eventually volunteers to lead a suicide fleet to draw the creature out of the kingdom.
I have the vague memory of "Fingalan Conspiracy" as the title, but haven't found anything remotely resembling that title.
Added- There is a pretty good chance it was a yellow cover gollancz series circa 1984, but I haven't seen anything similar on the gollancz lists I have seen online (anyone know of a definitive gollancz yellow cover list? - it was pretty much a guarantee of quality).