The Phylactery of Monstrous Attention is a very dangerous cursed magical item (it appears in the AD&D Dungeon Master's Guide, but I am really expanding that information by quite a bit).
This item appears as a blessed arm wrapping (or other form of jewelry), often bearing the symbol of a Deity (god) known to be good and helpful. Normal men can wear this item without triggering its effects.
When a priest of any devotion puts one of these on he should get ready for the biggest test of faith he will ever endure. This item is cursed, and draws the attention of supernatural beings, specifically the kind of supernatural beings that hold belief systems opposed to that of the priest.
This attention starts out small. The priest may encounter a few extra undead in the wilderness, and may feel watched while he is alone. Soon ghosts, lower planar creatures, and all manner of supernatural beasts will hound the priest on a regular basis, eventually even demons will get into the act. Most men end up dead within days, having renounced their faith as well, only a priest with a pure heart can hope to survive long enough to remove the phylactery (removal can only be done by a Wish spell, or by a Deity, your best bet is a Deity, since most worlds will have dozens of those, but only a handful of wizards capable of casting a wish spell).
The effects described above are mild compared to what happens when this item is worn by a truly powerful priest. When an Archbishop (or comparable level) dons this item he will attract the personal attention of one single Deity, specifically his own god's worst enemy. Just imagine if Satan himself took a personal interest in your life, now make it a lot worse because Satan is a pansy compared to most evil fantasy gods. Unlike the lesser creatures, the evil Deity will take his time, often keeping the priest alive for years and years (unless he renounces his faith, in which case he will be killed quickly so as to go to Hell/Hades/other evil afterlife).
This item is most commonly found on the body of the last person to wear it (commonly in tombs). It has been said by some scholars that this item was actually made by an ancient god to test the faith of his followers (probably why said god isn't around anymore).
This is only my interpretation of this item, which is based loosely upon its use in the older versions of the Dungeons & Dragons series of games. Your Phylactery of Monstrous Attention may be totally different, so always be sure and read the instructions.