I converted a plastic Belakor using a Vampire Count Varghiest and some Daemon Prince bitz:
Great Unclean One with Nurglings:
I decided to keep the diseased rat theme for my Nurglings.
Beasts of Nurgle/Plague Drones:
I oringinally converted these to be Beasts of Nurgle, but with 7th ed allowing the summoning of certain Daemon units, they can be used as Drones as well. Their bases are magnetized to be used in Fantasy as well.
Tzeentch Herald & Blue Horrors: