One option would be to forge your own blades. Take a long triangle-ish shape or plasticard, and carve it to look scythe like. Being that you are Nurgle, a little rot, rust, and decay will cover most modeling mistakes.
The hard part would be to find something to mount them on. When I made a scythe for a mounted wraith way back when, I was using a mini with a spear as the base, so it was an easy weapon head swap. The chosen look to have halberds, so that might work. I'm not that familiar with the modern chaos line unfortunately.
When I went to grab a picture, I noted that I had carved the blade using an old metal bit of sprue, not plastic. Shouldn't matter much. A skull from somewhere and greenstuff finished the blade. The base model is an old
DE dark rider with his face dremmeled out to make an empty hood. Not the best picture, sorry.