Vermintide 2 Wiki
Bah - These cults worshiping the dark gods always have the most pretentious names, don't they?
~ Olesya Pimenova

The Society of Harmonious Growth and Splendour was a local cult of Nurgle worshippers in Helmgart. They were under the command of Burblespue Halescourge and already active long before the invasion. They made preparations for the ritual encountered during Convocation of Decay and worked in the mines to prepare the production of Bile Trolls

Though not stated literally, this is probably the same cult as the "secret Nurgle cult" in Ussingen, which is mentioned by Olesya after completing Empire in Flames. This would mean Baron Justus Francke was allied with The Society of Harmonious Growth and Splendour.