Might this be Riddley Walker (1980) by Russell Hoban...?
From Goodreads:
In the far distant future, the country laid waste by nuclear holocaust, twelve-year-old Riddley Walker tells his story in a language as fractured as the world in which he lives. As Riddley steps outside the confines of his small world, he finds himself caught up in intrigue and a frantic quest for power, desperately trying to make sense of things.
From Wikipedia:
Roughly two thousand years after a nuclear war has devastated civilization, Riddley, the young narrator, stumbles upon efforts to recreate a weapon of the ancient world.
The novel's characters live a harsh life in a small area which is presently the English county of Kent, and know little of the world outside of "Inland" (England). Their level of civilization is similar to England's prehistoric Iron Age, although they do not produce their own iron but salvage it from ancient machinery. Church and state have combined into one secretive institution, whose mythology, based on misinterpreted stories of the war and an old Catholic saint (Eustace), is enacted in puppet shows.
This user review from Goodreads mentions that the perpertrators of the armageddon which led to this post-apocalyptic landscape are referred to as "Eusa."
The old tales and stories of our own lost time are perpetuated primarily through the existence of a modified Punch and Judy show. This puppet show is a government-sponsored propaganda machine wherein the main character is Eusa (a degraded and highly modified version of St. Eustace), a stand-in for the perpetrators of Armageddon, in which old knowledge and new superstition are mixed together to create a truly unique experience.