Creature — Illusion 1/5, 3B (4)
Common
{B}: Phantasmal Fiend gets +1/-1 until end of turn.
{1}{U}: Switch Phantasmal Fiend's power and toughness until end of turn.
Illus. Scott Kirschner

11/1/2005: Note that the wording "Effects that alter the creature's power alter its toughness instead, and vice versa, this turn" has been removed.
10/1/2008: Effects that switch power and toughness are always applied last when determining a creature's power and toughness, even if other power/toughness-changing effects are created later in the turn. For example, if you activate Phantasmal Fiend's first ability, it will become 2/4. Then if you activate its second ability, it will become 4/2. Then if you activate its first ability again, it will become 3/3.
Legal in Vintage (Type 1)
Legal in Legacy (Type 1.5)
Legal in Classic (MTGO)
Legal in Commander