Phenomenon is a card type used for Planar Magic. It is on oversized cards in the Planar deck and represent one-time effects players may encounter during planeswalking. Phenomena first appeared in Planechase (2012). 
From the Comprehensive Rules
(Battle for Zendikar
(September 26, 2015
- 310.1. Phenomenon is a card type seen only on nontraditional Magic cards. Only the Planechase casual variant uses phenomenon cards. See rule 901, “Planechase.”
- 310.2. Phenomenon cards remain in the command zone throughout the game, both while they’re part of a planar deck and while they’re face up. They’re not permanents. They can’t be cast. If a phenomenon card would leave the command zone, it remains in the command zone.
- 310.3. Phenomenon cards have no subtypes.
- 310.4. The controller of a face-up phenomenon card is the player designated as the planar controller. Normally, the planar controller is whoever the active player is. However, if the current planar controller would leave the game, instead the next player in turn order that wouldn’t leave the game becomes the planar controller, then the old planar controller leaves the game. The new planar controller retains that designation until he or she leaves the game or a different player becomes the active player, whichever comes first.
- 310.5. Each phenomenon card has a triggered ability that triggers when you encounter it. “When you encounter [this phenomenon]” means “When you move this card off a planar deck and turn it face up.”
- 310.6. A face-up phenomenon card that’s turned face down becomes a new object.
- 310.7. If a phenomenon card is face up in the command zone, and it isn’t the source of a triggered ability that has triggered but not yet left the stack, the planar controller planeswalks the next time a player would receive priority. (This is a state-based action; see rule 704. See also rule 701.21, “Planeswalk.”)