From Lexicanum

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Daemonicus Frateris was a pronouncement by a Inquisitor of the Imperium of Man that was declared by a member of the Daemonhunters.

It was used in cases where Humans are shown to openly worship the forces of Chaos. Ivixia Dannica declared it on Cimmaru Majoris against the primitive idol worshipping tribes on the western continent.

Sources

Specialist Games - Ivixia Dannica