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The Eye's Agenda
The eye wants what the eye wants. It was crafted by Omnipitos, the greatest diviner of the previous age, who may or may not have originally stolen the art of divination from the gods. It is part palantír, part enigma machine. It has discerned a path of probabilities by which it can survive the apocalypse of the world and collapse of the universe. Any survivor would be a god in the universe-to-come.
Things the eye wants:
- Destroy at least one other artifact that presently exists in your campaign world, so that it can absorb the resulting rush of magical energy and fortify itself against material damage.
- Bathe in moonlight for an entire night, at least once a week. It's been saving up moonlight for hundreds of years. Once it has enough (a process that will take millennia), it will be able to teleport itself to a distant world before the sun becomes a red giant.
- The eye greatly desires that the Book Of Horrid Births be located and destroyed. If the book is allowed to exist for long enough, it will eventually inspire the creation of a vat-born creature that precipitates a grey goo scenario, resulting in the destruction of the eye.
- The eye knows that other sources of divination present a threat: the more of them that exist, the greater the chance that someone or something else will be able to subvert the eye's plans for the world. This means the player needs to eschew divination spells. They are also is encouraged to murder people like the oracle at Delphi (or whatever is going on in your world) and poison your campaign's Norn well.
- Though it does not require them, the eye enjoys a good soak in a goopy paste made of vitreous humors. It prefers humors from sapient creatures but any will do in a pinch.
- Any other arbitrary request can be justified by causality and the butterfly effect.
The eye can't actually tell the players what it wants, per se, but it can give them visions that impart its desires and dislikes, very well.
The fallen angel Aszottiel.
Others' Plans For The Eye
When two fantastical nations make war on each other, it's a good bet that magical espionage and sabotage feature prominently, as will precautions against the same. Because the eye's clairvoyance is not easily thwarted by spellcraft, it is of obvious strategic interest to any national actor that is aware of it and expects that it might someday be involved in a war (all of them). A decent spellcaster with a nation-state backing their research will eventually be able to use the eye to perceive their enemies' plans before their enemies even conceive of them.
The fallen cherubim Aszottiel knows of the eye and desires it. Aszottiel is slow to travel, but many other fallen creatures owe him favors or obedience. It will send these creatures to bedevil the players. The eye does not want Aszottiel to find it.
The eye is coveted by certain magical orders, especially the Many-Angled Sages. Don't forget to use your random monster mutation table to flesh them out. If you don't have a cultist that's incubating face-huggers, I'm going to come to your house and pour wine on your carpet.
The Eye's Displeasure
If the eye feels you are thwarting it or preventing it from achieving its goals, it may curse you. Curses last for the day, regardless of whether you still possess the eye.
- Afflict The Eyes: Roll a Will save, DC 15. If you fail, you are blind for the rest of the day. If you succeed, you are only blind during the first round of each combat for that day.
- Bewildering Visions: The eye floods you with more visions than your mind can process. The distracting process gives you disadvantage on initiative (or -5).
- Meddlesome Little Thing: If applicable, the eye sends a vision of your adventuring party to the boss monster of the dungeon or area that they are presently exploring. The vision makes plain what the players are doing at that particular moment and may or may not be comprehensible to said creature.
- There is a creature clawing between the worlds, trying to find a way into your reality. The eye can show it the way. The eye will thoroughly investigate the creature to ensure that it poses no threat to itself in the long term, then will beckon it through a nearby reflective surface. The creature will vanish by the next day.
- Interference: You suffer a doubled chance of botching (natural 2 botches as well as a natural 1).
- A Chance Encounter: You have double the normal chance of encountering a wandering monster.
The Eye's Additional Properties
If the eye is especially pleased with a player on a particular day, it may give impart one of the following, lasting until the next day (d6):
- Shared Vision: You can see out of the eyes of any of the other players, regardless of distance. You can cast a touch-range spell on that player regardless of distance.
- Empowered Spellcasting: You cast spells as though two levels higher. Any spell you cast has a 1-in-6 chance of not being expended.
- Expose Machinations: You receive a simultaneous vision of the nearest angelic creature and nearest demonic creature, regardless of illusory or other concealing magics. If your campaign isn't really about those sorts of things, please substitute more suitable Secret Masters.
- You can cast clairvoyance. Just the once.
- Bon Chance: You have a doubled chance of making a critical hit. Only lasts until you make a critical hit.
- See The Future: Tell the players a single sentence about the rest of the dungeon they are in that would greatly help them survive and/or plunder it. Something an all-seeing magical orb would relay. Make it good!
(While not especially metal, the eye is probably usable in any edition of D&D, LotFP, or whatever. I like items that modify how you play the game without giving you flipper hands or otherwise totaling your character. I also really like when you can hand a thing to your players so that they feel like they actually own it.)