Image imported from derpibooru 
Original Description:
 
Not really getting off to a good start, Sacer Malum and Melicus Ostium were created against the will of their component mares with the exception being the stallion turned mare Sombra that was responsible for using the fusion potion that created the two fusions ultimately. With Sacer Malum despite being a three way fusion of Sombra (or Umbra if you so choose) Chrysalis and Nightmare Moon, was predominantly under the control of Sombra, the mare merging her self with the alicorn amulet to give herself a direct control and power advantage in the fusion overall much to the dismay of her other component mares. At the same time Melicus Ostium (or at least the siren mares that made her up) wasn’t necessarily on good terms with the changeling/nightmare/dark mare entirely either. Being ripped from another dimension (the EQG one) through the forced possession of random ponies bodies to then warp into their own siren forms only to be tricked and fused into a single large siren mare ain’t exactly ones idea of starting a unity. Sacer seeming more interested in using the siren as a pawn as opposed to an ally, even going so far as to possess the siren..
 
Though with their shared dislike of the Royals (or at least the ponies that made them up) one is curious to know as to whether or not their pseudo alliance will last….