Ratadrabik of Urborg and Boromir on the battlefield.
Steps
Activate Boromir by sacrificing it.
When Boromir dies, Ratadrabik triggers, creating a nonlegendary token copy of Boromir that is a 2/2 Zombie in addition to its other types.
Resolve the Boromir ability from step 1, causing the Ring to tempt you, and for the Boromir token to become your Ring-bearer, thus making it legendary.