Look into the old Fimir backstory - they were very cool, but not really explored these days. They were an Ogre-like race living in the marshes, with lots of rather unique ideas. They summoned fog and so forth. You can get models from Forgeworld, but you might be able to find some of the older plastic Heroquest models on eBay (or even old metals).
They might work as an order of Knights dedicated to fighting in the marshes, fighting against things like Fimir and bog octopuses and other marshy monsters.