As a MtG enthusiast, I'll tell you what I think of in terms of card artwork potential.



The creature itself clearly would fit the Horror type, possibly Demon Horror. It somewhat reminds me of Barubary from Breath of Fire 2. You say it was influenced by Black Phyrexian designs, and I love me some Phyrexia, but I actually see a bit more of Grixis in the design rather than Phyrexia, new or old. The primary reason I say this is because of the purple flesh, the eyes on the 2nd and 3rd heads, and lack of "jagged-ness". I'll address these in order:



Purple flesh: While not exclusive to Grixis, I find it more aligned with Grixis than Phyrexia. Why? Because it implies the creature has red blood. White Phyrexia features a lot of exposed muscle, however, and certain Phyrexian creatures such as Phyrexian Rager or Vatmother have a bit of purplish tissue as well. However, they aren't in as large quantity on the creatures, and the aesthetic I got overall was that Phyrexians only use blood as either a temporary material or vascular tissue to provide the creature with air supplies. Grixis is a place of horrors, to be sure, but they're pretty much purely organic horrors. In Phyrexia, the creature would have the Glistening Oil in place of blood, most likely, and at least some metal in place of flesh. If you ever redraw this fantastic piece, I'd recommend serrated, segmented metal plating of some kind.



Eyes: I recall an emphasis on the lack of eyes in New Phyrexian designs, Black or otherwise. This doesn't necessarily apply to compleated creatures like Glissa. You'll notice that of all the Praetors, only Vorinclex even has eye sockets, which seem to be empty. You'll see very, very few eyes in New Phyrexia, and I think the design notes that were posted on the Wizards website under the "Planeswalker's Guide to New Phyrexia" actually stated this outright. A similar-looking creature to this specimen, Massacre Wurm, lacks eyes, for example. Most Phyrexians, if I recall, navigate through other senses or by a sort of corporeal detachment that lets them see their bodies from a 3rd-person perspective.



Jagged-ness: While the creature includes spikes, a few seemingly-vestigial limbs, and even shoulder-claws of some sort, it feels a bit too "smooth" and purely organic. Skinrender aside, Black Phyrexians typically feature a lot of deadly serrations, segments, sinews, pistons, etc. I'll once again cite Massacre Wurm, which is an excellent example of this "jagged" design. Even exceptions like Phyrexian Obliterator feature an onslaught of claws and bladed spinal tentacles to make the creature pointy in every direction. If I had to give you a design rule of thumb, a Black Phyrexian's entire body should theoretically be capable of killing something if separated and used as a weapon.



As for the overall art itself, I think it suffers from the Blue spell being so emphasized. If this were card art spoilered for a set, I wouldn't be sure if this was a creature spell or an instant meant to deal with creatures. Perhaps it could have been the creature breathing a caustic, ideally Infectious condensed gas "beam" at the mage, while the mage struggles to hold it back with a defensive spell?



A fantastic piece to be sure, and I hope you get a gig with Wizards, but there are a few small things that I think they'd ask you to revise if you submitted this as a Black Phyrexian creature. If I had to guess what stats it'd have as a Black Phyrexian creature, it'd be something like this:



Narcturias, Behemoth of Night (4BB)

Legendary Creature - Horror

Infect, Trample

When Narcturias, Behemoth of Night attacks, redistribute any number of -1/-1 counters among all creatures.

5/4