Nazgûl enters, triggering its first activated ability, causing the Ring to tempt you; choose Nazgûl as your Ring-bearer, making it legendary.
Nazgûl's second activated ability triggers, putting a +1/+1 counter on each Wraith you control.
Activate Altar of Dementia by sacrificing Nazgûl.
Ratadrabik of Urborg triggers, creating a token copy of Nazgûl that's a 2/2 Zombie.
Resolve the ability from step 4, milling target player equal to the sacrificed creature's power.
Repeat from step 2.
This loop turns your Ring-bearer into an infinite milling machine by exploiting Ratadrabik of Urborg’s ability to replace fallen legends with nonlegendary zombie copies. Because the Ring tempts you when Nazgûl enters, it makes your Ring-bearer legendary—allowing Ratadrabik to catch it when it hits the graveyard. Meanwhile, the growing threat of each Nazgûl pumps its own power with +1/+1 counters via the Ring-tempting mechanic, while Altar of Dementia instantly converts that escalating power into an unstoppable cascade of milled cards.