Karn, Silver Golem
Legendary Artifact Creature — Golem
Whenever Karn blocks or becomes blocked, it gets -4/+4 until end of turn.
: Target noncreature artifact becomes an artifact creature with power and toughness each equal to its mana value until end of turn.
Mark Zug
Rulings
A noncreature permanent that turns into a creature can attack, and its {T} 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.
(wotc • 2008-08-01)
The blocking ability triggers only once no matter how many blockers are declared.
(wotc • 2004-10-04)
Rulings
A noncreature permanent that turns into a creature can attack, and its {T} 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.
(wotc • 2008-08-01)
The blocking ability triggers only once no matter how many blockers are declared.
(wotc • 2004-10-04)