Ixidron

{3}{U}{U}Creature — Illusion*/*

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.

Turning a face-down creature face-down typically has no effect; the creature's status is unchanged. (2006-09-25)
If Ixidron and another creature are entering at the same time, the other creature enters face up. (2006-09-25)
The 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. (2006-09-25)
You 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. (2006-09-25)
Creatures 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. (2018-04-27)