Polymorphist's Bane

At the beginning of your upkeep, create a 1/1
green Ooze creature token.
When another creature card enters the battlefield,
if it wasn't cast, you may have Polymorphist's
Bane become a copy of that card except its name
is still Polymorphist's Bane, it's legendary in
addition to its other types, and it gains this ability.
"It didn't consume the dragons, it BECAME one."