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.
Repeat.

Infinite death triggers

Infinite ETB

Infinite LTB

Infinite sacrifice triggers

Ratadrabik of Urborg

Boromir, Warden of the Tower

Activate Boromir by sacrificing it.
When Boromir dies, Luminous Broodmoth triggers, returning Boromir from your graveyard to the battlefield without a flying counter on it due to Solemnity.
Resolve the Boromir ability.
Repeat.

Solemnity

Luminous Broodmoth

Activate Boromir by sacrificing it, giving creatures you control indestructible until end of turn.
Activate Krark-Clan Shaman by sacrificing an artifact, dealing 1 damage to each creature without flying.
Sonic's second ability triggers for each creature you control with haste or flash that was dealt damage, creating that many tapped Treasure tokens.
Repeat from step 2.

Infinite tapped Treasure tokens

Infinite damage to creatures

Sonic the Hedgehog

Krark-Clan Shaman

Activate Boromir by sacrificing it.
When Boromir dies, Yedora triggers, returning Boromir from your graveyard to the battlefield face-down as a Forest land.
When Boromir enters the battlefield, Zimone triggers.
Holding priority, activate Boromir by tapping it, adding {G}.
Resolve the Zimone trigger, turning Boromir face-up.
Resolve the Boromir ability from step 1.
Repeat.

Infinite green mana

Infinite landfall triggers

Zimone, Mystery Unraveler

Yedora, Grave Gardener

Cast Boromir by paying its mana cost if it is in your hand. Otherwise, activate Tayam by paying {3} and removing three counters from creatures you control, causing you to mill three cards and return Boromir from your graveyard to the battlefield with a vigilance counter on it.
When Boromir enters the battlefield, Metastatic Evangel triggers, causing you to put a +1/+1 counter on Cryptic Trilobite and Basking Broodscale and a vigilance counter on Boromir.
Basking Broodscale triggers, creating a 0/1 Eldrazi Spawn creature token that enters with a vigilance counter on it.
Activate Cryptic Trilobite by removing a +1/+1 counter from it, adding {C}{C} that can only be spent to activate abilities.
Activate the Eldrazi Spawn by sacrificing it, adding {C}.
Activate Tayam by paying {3}, removing both vigilance counters from Boromir and another counter from a creature you control.
Holding priority, activate Boromir by sacrificing it.
Resolve the Tayam ability, causing you to mill three cards and return Boromir from your graveyard to the battlefield with a vigilance counter on it.
Repeat from step 2.
Repeat.

Combos with "Boromir, Warden of the Tower"