Cheapest priced copy (Commander 2019): $0.49 at Card Kingdom, near mint, their price list as of .
Ixidron 
Creature — Illusion · */* · Rare · Commander 2019 · #87
As this creature enters, turn all other nontoken creatures face down. (They're 2/2 creatures.)
Ixidron's power and toughness are each equal to the number of face-down creatures on the battlefield.
Legal inCommanderModernLegacyVintage
Not legal inBrawlStandardPioneerPauper
3 printings · $0.49 to $8.99 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Commander 2014 | 116 | Rare | $0.69 | — |
| Commander 2019 | 87 | Rare | $0.49 | — |
| Time Spiral | 65 | Rare | $0.99 | $8.99Out of stock |
5 rulings from Wizards of the Coast
- 25 September 2006Turning a face-down creature face-down typically has no effect; the creature's status is unchanged.
- 25 September 2006If Ixidron and another creature are entering at the same time, the other creature enters face up.
- 25 September 2006The controller of a face-down creature can look at it at any time, even if it doesn't have morph. Other players can't, but the rules for face-down permanents state that "you must ensure at all times that your face-down spells and permanents can be easily differentiated from each other." As a result, all players must be able to figure out what each of the creatures Ixidron turned face down is.
- 25 September 2006You turn the creatures face-down *as* Ixidron enters. There is never a moment when Ixidron is on the battlefield and the creatures are face-up. If a creature on the battlefield has a "whenever another creature enters" ability, it won't trigger because that creature will be face down before Ixidron enters.
- 27 April 2018Creatures turned face down by Ixidron are 2/2 creatures with no text, no name, no subtypes, no expansion symbol, and no mana cost. These values are copiable if an object becomes a copy of one of those creatures, and their normal values are not copiable.