Cheapest priced copy (Commander 2014): $0.99 at Card Kingdom, near mint, their price list as of .
Xathrid Demon 
Creature — Demon · 7/7 · Mythic · Magic 2010 · #122
Flying, trample
At the beginning of your upkeep, sacrifice a creature other than this creature, then each opponent loses life equal to the sacrificed creature's power. If you can't sacrifice a creature, tap this creature and you lose 7 life.
Legal inCommanderModernLegacyVintage
Not legal inBrawlStandardPioneerPauper
2 printings · $0.99 to $5.99 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Commander 2014 | 170 | Mythic | $0.99 | — |
| Magic 2010 | 122 | Mythic | $1.49 | $5.99 |
2 rulings from Wizards of the Coast
- 1 October 2009If you have two Xathrid Demons, both abilities will trigger at the beginning of your upkeep. When the first Demon’s ability resolves, you can sacrifice the second Demon to satisfy the requirement (“Sacrifice a creature other than Xathrid Demon” really means “Sacrifice a creature other than this creature”). When the second Demon’s ability resolves, you must sacrifice another creature. If you can’t (because the first Demon has somehow left the battlefield and you control no other creatures), you lose 7 life, regardless of whether the second Demon is still on the battlefield at this time.
- 1 October 2009When the ability resolves, if you control any other creatures, you must sacrifice one of them. You lose life only if no creature can be sacrificed.