When Rangers of Ithilien's ability triggers, you can choose not to target a creature just to have the Ring tempt you. However, if you do choose a target, and that target is illegal at the time the ability tries to resolve, the ability won't resolve and none of its effects will happen. The Ring won't tempt you.

Rangers of Ithilien's triggered ability lets you gain control of a creature if it has lesser power than Rangers of Ithilien at the time that you choose the target and at the time that the ability resolves. After that, it no longer matters what the creature's power is; you will continue to control it for as long as you control Rangers of Ithilien.

If you gain control of a creature with the triggered ability, you can immediately choose that creature as your Ring-bearer as the Ring tempts you.

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.

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.

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.

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.

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

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

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.