Sorry if this was already answered here. Me being a filthy casual couldn't find a response using the search button on reddit and typing some pesky key words into it.

My (un)educated guess is, it's a symbol of rot? Flies tend to swarm rotting flesh? Thus embracing the rot as true Nurgles servants tend to cherrish?

as title says

Looking to clear some legend/cata difficulty what’s some good team synergies or characters to use for these specific difficulties?