You should definitely work out why the enemy wasn't using the ring, whether it would be dangerous for them, they only just obtained it and had not yet had time to remove it from the box it was transported in, etc.
Dug up some notes for a couple of evil rings from old games. These were made for 3rd/3.5th so if you are using 4th you may need to tweak them.
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Artifact - Seven Soul Ring.
This silver ring is set with seven flawless sapphires that each glow with an inner light(If worn openly imposes a -2 penalty to hide checks at night or in dark/shadowy locations, provides sufficient illumination for careful movement in the dark). If the wearer is subjected to any instantaneous death or level drain attack, spell or ability, and at least one gem is lit, the effect is negated and the light in one of the gems gutters out.
It is unknown who or what crafted the ring but it is a creation of great evil. Each gem holds the trapped soul of a sapient being of good or neutral alignment. When a charge is expended one of the trapped souls is permanently destroyed and cannot be raised by any non-divinity(A divinity must posses either the Gift Of Life or Life And Death salient divine abilities in order to restore the being). In order to recharge the gem a suitable being must be ritually murdered in a desecrated area. The being must be restrained and unable to avoid attack, when slain by the wearer of the ring the soul of the victim is trapped as by Trap the Soul with no save or spell resistance and with no concern to gem value required to hold the soul.
Paladins or other classes who are obliged to maintain a code of good who utilise the ring without being aware of it's nature must seek immediate atonement or fall, losing access to class abilities. If aware of the nature of the ring they must act to prevent it's use or fall.
The ring cannot be destroyed by any normal methods, as with other artifacts precise requirements for destruction are left to DM. Detects as Strong Conjuration, Strong Necromancy, Overwhelming Evil.
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Cursed ring - Kings Bane
This artfully decorated gold ring is set with a single large ruby surrounded by a number of small diamonds. It has been used multiple times since its crafting in the assassination of Nobles and Kings. Frequently given as a "gift" to the target, it provides protection against melee attacks granting a +4 deflection bonus to AC, damage resistance of 5/- and the wearer is granted the effect of the Endure Elements spell while worn(wearer remains comfortable in any normal temperature range but does not gain any resistance to fire/cold damage attacks).
However the ring is cursed, firstly with a mild compulsion not to remove the ring(will save DC 17 when attempting to remove the ring, failure results in justifying to self a reason not to remove it and cannot make another attempt that day). Secondly the wearer suffers a -4 penalty to fortitude saves verses poison or disease. Thirdly against any ranged attack the deflection bonus and damage resistance normally granted by the ring do not apply, the attacker receives a +4 circumstance bonus to hit, any potential critical hit receives +6 to the roll to confirm the critical and the damage of the ranged attack is doubled.
Detects as Moderate Abjuration, Faint Divination, Faint Enchantment, Faint Necromancy
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Ring of [Alignment]
[Alignment] is one of Balance(true Neutral), Chaos, Evil, Good, Law
If worn by a character that matches the rings alignment they gain 1 effective level causing +1 on attack rolls, saves, skill checks, ability checks, caster level and effective level(for determining the effect of class abilities that scale by level). Casters gain 1 spell slot at the highest level they have available.
If worn by a character that has an opposing alignment to the ring they gain 1 negative level causing -1 on attack rolls, saves, skill checks, ability checks, caster level and effective level(for determining the effect of class abilities that scale by level). Casters lose 1 spell slot at the highest level they have available.
Balance rings treat the four extreme alignments(LG, CG, LE, CE) as opposing for causing negative levels.
The effects can only be removed by removing the ring, some rings may be cursed so they cannot be removed by those of an opposing alignment without the use of a remove curse spell.
Detects as Moderate Conjuration, Moderate Necromancy, Moderate [Alignment]
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