Viriconium (series of novels)

Viriconium is a fictional city created by M. John Harrison and also the name of the cycle of novels and stories set in and around it.
Esteban Rivera, 27 Augustus 2010, quoting from the English Wikipedia

The series is part of the "Dying Earth" subgenre of science fiction, popularized by Jack Vance in the 50's and characterized by a setting in the very far future, in the twilight of the planet, usually in decadent, foundering societies haunted by past glories. Harrison brought to this a postmodern touch, subtly shifting times, events, and places for a certain paradoxical dreamlike quailty — as he says, «Viriconium is never the same place twice.»
Eugene Ipavec, 16 February 2006

Viriconium

(reconstruction)

image by Eugene Ipavec and Vincent Morley, 16 February 2006

The short story in which the flag of Viriconium is "The Luck in the Head." Relevant quote:

It was the anniversary of the liberation of Uroconium from the Analeptic Kings. Householders lined the steep hill up at Alves. Great velvet banners, featuring black crosses on a red and white ground, hung down the balconies above their naked heads.

(pg. 394, Millennium/Gollancz omnibus edition, 2000) With a description this vague any number of interpretations are plausible, especially as the text leaves it unclear wheter a plural of banners or crosses is meant (I'm inclined to interpret it as the latter). (The city is at this point named Uroconium, and is further renamed to Vira Co at the end of the story.)
Eugene Ipavec, 27 Augustus 2010

Harrison is English, so I have taken the liberty of borrowing the English flag.
Eugene Ipavec, 16 February 2006

According to the frontispiece, the author adapted "The Luck in the Head" into a graphic novel in 1991 (ISBN: 0-575-05014-4); a definitive design of the flag might appear there.
Eugene Ipavec, 27 Augustus 2010

(other possible interpretations)

Alternative reconstruction #1

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The Northmen

(reconstruction)

image by Eugene Ipavec, 16 February 2006, based on cliparts from heraldicclipart.com

Tribes with a long-term grudge against Viriconium that take part in a bloody dynastic war in the novella The Pastel City. The flag is not described in one place, but can be reconstructed from fragmentary descriptions scattered about the piece:

The arms of the pretender to the Viriconese throne, Canna Moidart, are given (on a coin) as «wolf's head beneath three towers».
The wolf's head is described as «wine-red» and as being on a black background in the form of a painted badge on a Northern aircraft at the end of the novel.
The use of these arms on a banner is mentioned during the battle between Lord Waterbeck's troops and the Northerners outside Duirinish.

Eugene Ipavec, 16 February 2006