The Culture was aware of the Issorilians many centuries before Idiran-Culture War and sought some way to improve the Issorilians behaviour. The civilization was not only aggressively expansionist, but had a culturally and socially ingrained traits for exploitation, sadism and brutality.
Maintaining contact and relations was difficult since the Issorilians treated technologically more advanced civilizations with suspicion, and the reverse with contempt. Eventually, the Culture found the Padressahl in the same volume of space as the Issorilians; the Padressahl had a similar evolutionary history and physiology as the Issorilians and so more likely to be accepted, but they also were closer to the Culture in terms of moral outlook. The Culture appealed to the Padressahl to guide the Issorilians.
The Padressahl accepted. Early attempts were rocky. During one episode a Padressahl trade delegation was eaten, prompting them to nickname the Issorilians the Affront. Ironically, what was intended as insult was enthusiastically embraced by the Issorilians.
However, a century or so after the Idiran-Culture War the Padressahl suddenly sublimed. The Affront was freed to harass neighbouring civilizations, including those being shepherded by the Culture and others everyone had not seen fit to contact yet