Macguffin suggestions!!!

So I'm still a new DM (mature player that only started playing a couple years ago), and I think my first campaign finally has a finale... however, I need a MACGUFFIN to tie it all together.  Here's the setup (feel free to borrow this if you're inspired):

After breadcrumbing a mystery of "the sins of the father" and even leading toward one PC's father being its root, it turns out that about 20 years ago, all FIVE of the PC's fathers knew each other.  They were hired separately, never even learning each other's real names, to find and retrieve an ancient artifact.  After working for weeks (months?), and using each of their skills, they obtain said artifact.  But one of them quickly realizes the artifact's immense power is too great or evil to be possessed by anyone, so the fathers agree to hide it, and disappear, keeping the secret among them.

So, here's the question: What is a suitable artifact (official or homebrew) that should not be possessed by anyone yet would be coveted by a truly evil person?

The closest I found might be Wand Of Orcus, but I don't know if that is 'evil' or high stakes enough?  Any suggestions are appreciated.

The Wand of Orcus is probably plenty, but if it's not, create your own demon (you really only need a name and maybe a bullet or two of lore) and then make your demon's version a souped up ring/gem/shield/horn/crown/whatever of the Wand of Orcus. You could do this in the Homebrew tool by starting with the Wand of Orcus and going crazy on it's current stats while throwing in a few more. You could also just make a stat block for describing it if no one is ever going to wield it.

So, I would suggest thinking about 'the One ring' from Lord of the Rings here. The powers themselves are somewhat nebulous right? A mortal can become invisible sure, but the slow corruption of the person possessing it? The ability to control the other rings. It's all pretty poor explained when you actually look at it. 

A good evil macguffin generally doesn't even need its powers explained. What you do need for an evil macguffin is good examples of what happened to those who went before. So for Lord of the Rings we've got the rise of an evil villain who nearly conquered the world, we've got the corruption of other ring bearers particularly the Nazgul, and we've got the development of Smeagol into Gollum. Through the journey we see that this harmless looking ring has done. 

In fact it's often when a Macguffin is explained (what it can do, why it must not fall into the wrong hands) that things can kind of fall apart. Think the Death Star plans in Star Wars. We know the plans contain a weakness which is great, but once that weakness is explained it's kinda silly at it's core. So here's the question why does it need to be a formally written item? One of my groups had a Pendant that was the evil Macguffin which every so often would cause all sorts of wild magic chaos until they realised it needed to be destroyed. It got added to a player's character sheet as a custom item 'Golden Pendant with Obsidian Gem'.

Likewise you could also go the Marvel direction. The infinity gems were capable mostly of whatever the writers needed them to be capable of. Their powers weren't from memory all that explicitly stated and their powers definitely have changed over the years. So you could just have the item be capable of whatever you decide in the moment. Even then, just add a very plain custom item to the character sheet 'mysterious gem'.

In the event that you're worried about the identify spell, I've got two strategies for you.

1. Identify simply doesn't work on this macguffin. WotC set the precedent for this over and over again. In fact Vecna: Eve of Ruin did this a few years ago. They created a disguise item that no spell could effectively see through. It might seem unfair, but it's a legitimate tactic at your disposal. 'Dark magic clouds this item, making it impossible for you to see the truth of its powers'.

2. Be unspecific about the answers you give when responding to identify. As written, Identify doesn't give you the name of the item. Nor does it give you all that much if you interpret the wording more generally. So in responding to what 'properties' a magic item has, we could say: 'when the item is triggered, it casts a spell. The trigger is the word Donkey. It has 12 charges which reset at dawn'. It's a literal but valid way of interpreting the spell identify. [Side note, the line about 'You learn whether any spells are affecting the item and what they are. If the item was created by a spell, you learn which spell created it.' does not mean you know what spells can be cast from the item, rather you learn about a spell which has been cast upon an item. Has a Ward or Guard been placed upon the door? It's not everyone's interpretation, but it is a valid reading of the text]

@BigHeartedGuy Thanks for the reply.  I was looking more along the lines of "what the Macguffin could do", but I like the input.  The idea of starting with the Wand Of Orcus and tweaking it is a great idea.  I'll think about it.  Thanks!

@martintheactor I like the Infinity Stone idea of a vague but powerful ability.  A power not evil in itself, but can be abused in the wrong hands.  Definitely a cool direction.  Thanks!

To go with what Martin said some of the greatest macguffins in fiction are never explained. Ronin is an amazing movie with several deadly factions fighting to possess a brief case, I'd watched it two or three times before I realised you're never told what is in the brief case or why everyone wants it. Similarly the Rabbit's Foot in Mission Impossible 3 was never explained (at least not until a retcon two decades later in Final Reckoning) and it's still a great film. One of the most famous is Pulp Fiction with another briefcase everyone wants but we're never told what's in it. Even Hitchcock, who invented the term, had several films where he left the central mystery as a mystery. Drop some vague hints and never explain it and if you do it right your players will be guessing long after the campaign finishes

You can always go a little less ostentatious.

Think the Holy Grail from Indiana Jones. The templars hid it so that nobody could become immortal. Immortality would be a good, simple one which puts more emphasis on why the other characters they are fighting are the wrong sort of people, rather than putting the emphasis on what they can do with the macguffin.

There are a great many ultimate evil type artefacts, I may be over stepping but is part of it a temptation to use for the greater good you want or am I reading that wrong?

The thing about mcguffins in a story is the characters in the story care about the mcguffin, but the audience doesnt. The briefcase in Pulp Fiction basically took the trope to its finale conclusion: the audience never knows whats in the briefcase, but in the story people die fighting for the briefcase.

In dnd mcguffins are basicaly story hooks. Like players are hired to guard a convoy from bandits. The convoy isnt all that important to the players, but it does get them engaged in te game. If you want to take a mcguffin and tie it more intricately to the plot, usually with some sort of unexpected twist, then things can get a little more complicated. Surprise! The convoy was a ruse to deliver the party to some bbeg who want to kill the party for something they did months ago.

In the 2014 DM's Guide almost every artifact in their fits under the inherently evil category or causes great destruction in the wrong hands category, because artifacts are inherently powerful and therefore can do powerful things. Evil Artifacts: Aforementioned Wand of Orcus, Book of Vile Darkness, Sword of Kas, Hand and Eye of Vecna (Vecna supposedly made all of these but the wand, so maybe look into him as a source of creation for evil things in particular). All tempt their wielder into darkness and evil. An Orb of Dragonkind could destroy civilizations by strolling into the capital and activating it. All you need for the artifact is the power to kill and destroy. Anything you can think up.

Scroll of Tarrasque Summoning is not an artifact, but an interesting way to raze a civilization.

Don't have a specific set of powers, that just invites the party to figure out a way to make it usable / useful. Instead I'd suggest having multiple NPCs tell the party contradictory / partial information about it. I mean the whole premise of the story is this item has been hidden because even just a little of its power scared the fathers into hiding it. So why should anyone know what it really does? Surely there would just be legends or rumours about it. Even the fathers might only know a couple of things it can do. 

Some examples of "so powerful it should never be used" powers that might be mentioned as what the thing can do:

1. Time travel - the ability to change the past and prevent your enemies from ever being born...
2. Unlimited mind-control - the ability to force anyone to do what you tell them to.
3. Destroy gods / Destroy the Weave that powers all magic
4. Summon titans or tarrasques or raise armies of undead.
5. Control life & death itself

Note that the issue doesn't necessarily have to be the artifact itself (most of which are powerful but, frankly, not worldshaking). For example, in my campaign there's a door that can only be opened by the Hand of Vecna, so the real goal is the door.

If you want magic items that just shouldn't be used, try an orb of dragonkind.