If a spell is cast with an alternative cost that includes mana (such as flashback), Boromir's second ability won't trigger and won't counter that spell. Similarly, if a spell is cast without paying its mana cost, but the player did pay mana for additional costs such as from kicker or from Thorn of Amethyst, Boromir's second ability won't trigger and won't counter that spell.

If the creature you choose as your Ring-bearer was already your Ring-bearer, that still counts as choosing that creature as your Ring-bearer for the purpose of abilities that trigger "whenever you choose a creature as your Ring-bearer" or abilities that care about which creature was chosen as your Ring-bearer.

Players may cast spells that they know Boromir, Warden of the Tower will counter. Any abilities that trigger when spells are cast will still trigger and resolve if appropriate, and any effects that count how many spells are cast will still count those spells if appropriate.

Each player can have only one emblem named The Ring and only one Ring-bearer at a time.

The Ring gains its abilities in order from top to bottom. Once it gains an ability, it has that ability for the rest of the game.

The Ring can tempt you even if you don't control a creature. In this case, abilities that trigger "whenever the Ring tempts you" will still trigger.

Some spells and abilities that cause the Ring to tempt you may require targets. If each target chosen is an illegal target as that spell or ability tries to resolve, it won't resolve. The Ring won't tempt you.

Each time the Ring tempts you, you must choose a creature if you control one.

As the Ring tempts you, you get an emblem named The Ring if you don't have one. Then your emblem gains its next ability and you choose a creature you control to become (or remain) your Ring-bearer.