But according to Metamox, a site with copious amounts of info on Magic cards, Xathrid Demon is "unplayably bad." Still, there is some demand, especially for foils. In July 2015, popular online Magic card retailer StrikeZone Online was buying them for almost $6. This is odd, considering that most retailers would sell Xathrid Demon foils for about $3. So did StrikeZone know something that the rest of us don't? Is this card actually more valuable than the majority of dealers and players believe?
Xathrid Demon does have some interesting things going for it. Not only is it a 7/7, but it flies and has trample. At the beginning of each of your upkeeps, you must sacrifice a creature other than Xathrid Demon. Each opponent loses life equal to the sacrificed creature's power. If for some reason you can't sacrifice a creature, you tap the Demon and lose 7 life.
The upside of a card like this is that there are plenty of reasons to sacrifice creatures in a Mono-Black deck. With cards like Grave Pact, Dictate of Erebos, and Butcher of Malakir hurting your opponents by forcing them to sacrifice a creature whenever you sacrifice a creature, this Demon has definite utility.
Mono-Black also has creatures like Mikaeus the Unhallowed, which gives creatures undying, so you can reuse the creatures you sacrifice. There are plenty of other ways to make tokens every turn that you can sacrifice. So most of the time, you can make the Demon do a great deal of work for you. That, and the Demon can deal a good amount of damage on its own.
It would seem to me that MetaMox greatly undervalues this card. Being a Demon, which is a very popular tribe in Magic, Xathrid Demon will have its fans just for collectible value. There are some interesting applications for this card, especially in Commander.
I'm not sure that there's a ton of money to be made on this card. But, since foils can be had pretty cheap, it's not a bad idea to see what your local buylists will pay for them and see if you can pick them up cheaper. It's probably doable. Most people consider it bulk, but to the right players, especially in foil, it's definitely a useful piece.