Xenic Poltergeist
Masters Edition IV — uncommon
Creature — Spirit
Xenic Poltergeist rulings:
2008-08-01: A noncreature permanent that turns into a creature can attack, and its abilities can be activated, only if its controller has continuously controlled that permanent since the beginning of their most recent turn. It doesn't matter how long the permanent has been a creature.
2004-10-04: Wears off at the beginning of the upkeep, not during upkeep.