Excalibur, Sword in the Stone

Excalibur is all colours.
At the beginning of your end step, if you control
a creature with power 7 or greater, Excalibur
becomes a Legendary Artifact Equipment with "Equipped creature gets +5/+5 and has Menace, Double Strike, and Trample." Attach it to that creature.
Whenever equipped creature dies, Excalibur
becomes a land and is no longer an artifact.