While at first glance this item would appear to be so convoluted in it's creation and use that it would not seem to be worth introducing into a campaign, the amazement that the party might have after seeing it used would be worth almost any setup!
Only a few of these would likely exist in any campaign setting, but once you are in a high-level campaign, their use would almost be a certainty. They can only be made by the skill of high level wizards with an Illusionist in tandem or by a very high level wizard with also high levels in Illusionist. The primary effect of this amulet is to make the bearer immune to one specific type of magical effect, if said effect would kill or otherwise fully disable the bearer.
Damage or effects are wholly negated, and a specialized programmed illusion triggers at exactly the same moment to give all viewing the event the impression that the attack or effect was done with one hundred percent efficacy. No save is mentioned in the description, and I would be inclined to rule non is possible unless magic is used to ensure that the bearer was actually killed or disabled. The description says that direct tactile confirmation is the only thing that would make the wizard or creature be able know that their attack was unsuccessful. Further, the bearer is made invisible during the trigger process, so the bearer could move away from the scene.
What an amazing chance to pull off some amazing capers by an inventive party, if they obtained this item!
Even more amazing, some NPC or enemy creature using this device on the players once it becomes known that they commonly use some specialized attack (fireballs or the like). Even better once the enemy was formally defeated by said attack, but somehow lives to come back for revenge!
Tip:
Using something like this, while requiring extensive planning and careful background understanding, can really make for game sessions that will be talked about for years afterward. Consider something like this when world-building in your campaign.