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 Yahenni 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, giving Yahenni indestructible until end of turn.
Repeat from step 2.
This loop transforms a simple wraith into an unstoppable engine of endless sacrifices and tokens. With Ratadrabik of Urborg on the battlefield, any legendary creature's demise is met with a non-legendary 2/2 Zombie copy, which is the exact trick that lets us recycle Nazgûl over and over. Every time a fresh Nazgûl enters, it triggers the Ring to tempt you—making that Nazgûl legendary—which instantly pumps your Wraiths and feeds Yahenni, Undying Partisan with a steady stream of sacrifice fodder. You can then use Yahenni to sacrifice the Nazgûl, triggering Ratadrabik of Urborg all over again to restart the loop infinitely while keeping Yahenni safely protected with indestructible.