Deathly Hallows
Paulum Mortis
Three Epic Level magic items linked together, created in the thirteenth century by the Peverell brothers: Antioch, Cadmus, and Ignotus... the Elder Wand, the Resurrection Stone, and the Cloak of Invisibility.
Tier 1
The true Cloak of Invisibility makes you invisible, the Elder Wand makes the wielder undefeatable in battle, and the Resurrection Stone allows a person to communicate with their dead.
Tier 2
This isn't what any of the three artefacts actually do.
The Cloak makes its Master unseen, undetectable, and unfindable. Employed by someone else and it'll work just like any shroud made from demiguise hair, or imbued with a badass disillusionment charm: it's just an invisibility cloak. When its Master—one of the Potter line—uses it though, the wearer wouldn't have to worry about something stupid like her knees and feet being visible.
The Stone doesn't actually yank people out of the afterlife… I mean, obviously? It can provide a shade of memory though, and whether or not that memory is eventually going to convince you to commit suicide is something of a debate. It can also aid in the creation of low‑cost Inferius—which is what Grindelwald wanted it for—and can bind ghosts to do your bidding. A classic artefact of Epic Necromancy.
The Wand though is the most interesting, as it doesn't actually make you any more powerful than you already are. Its wielder can't for example, smash mountains to rubble with a wave of her hand, not if she couldn't do that to start with anyway. Elder Wand is a communication shard. Magical foci are kinda sentient right? "The wand chooses the wizard, Harry," and choice implies a measure of cognition—even plants are big fans of sunlight, they choose to go towards it, and everyone agrees plants are alive. And when a wizard uses their wand, they're communicating to it whatever it is they want to get done, apply magic through the channel and BAM! magic happens.
So what the Elder Wand is doing—the edge its offering the wielder—is shockingly simple: its communicating directly to you, whatever your opponent it communicating to their wand. Like eavesdropping maybe? Spying. You get the few fractions of a second it takes your enemy to cast his spell, with full—if instinctive—knowledge of what they're about to do, while you get to decide how to act to counter it.
If you've ever so much as been on a sparring mat, this tickle at the back of the mind is one unassailable advantage.
Also, wands gain experience alongside their wizards. They become more in tune with what they're expected to do as the wizard herself gains in power and practice. Elder Wand has been killing people a lot for centuries, in the hands of every tin-pot wannabe Dark Lord for the past seven-hundred years: it has a lot of experience. Seems clear casting Fiendfyre through Elder Wand will be a lot less difficult for you than attempting it for the first time with the wand you got from Ollivander, way back when you were eleven years old. A lot more 'sympathy' and a lot less 'slippage'.
Tier 3
Deathly Hallows and Killing Curses: Avadakedavra!
The question as to whether or not thestrals are immune to Killing Curses remains academic until I can slaughter one for science—though I strongly suspect they are—what we do know is the damned things don't have hair! Making a wand from thestral hair is like making one from hen's teeth. Why not pick me up a jar of tartan paint while out gathering ingredients, eh? Just don't be surprised when the shopkeeper offers to trade you for a "long weight."
Thestrals were involved in the creation of these objects though, 'cos you know: "Deathly" Hallows, and thestrals are visible to anyone who's seen death and understands what it means—looking at you Harry, didn't you just murder that Quirrell guy?—This is where the Cloak's invisibility power comes from.
All three items can render the wizard wielding them immune to Killing Curses.
It's doubtful many, if any bearers knew the Elder Wand could produce a counter‑curse or magical shield capable of blocking Avadakedavra, but it can, its Death's own wand after all, ain't it? And you can always defend—both best and worst, against oneself.
Cloak of Invisibility grants its Master, or anyone shielded by its Master, the ability for any Killing Curse to simple pass straight through them, as though they weren't even there. Just like magic.
Resurrection Stone is much simpler. An artefact of High Necromancy, it'll just absorb the hatred and malice necessary to power a death spell, it'll glow green or whatever, making the bearer untouched looking like a boss!
Tier 4
The triumvirate each have different powers, but they are equal to one another too.
Elder Wand can cast a spell—say on a spinning magical eye—enabling a piercing of the Cloak.
Cloak makes it so its Master, once comfortably inside her, appears as if they're experiencing a little death—and so three turns of the Stone will render the invisible person visible. Though I guess you'd have to call out her name? It's polite after all.
And any ghosts commanded with the Stone will be incapable of finding Cloak, nor coercing Wand.
Wearing Stone—on your ring finger?—will confuzzle the communication aspect of Elder Wand, so all spells cast—from Tickling Charm to Expelliarmus—will feel like Killing Curses.
Whereas Cloak will naturally mask the communication aspect with non‑presence.
One, two, three: immune, immune, immune.
Tier 5
The question remains however, what bonus is achieved from earning the trifecta. Unification of all three leading to something greater than the sum of their parts. Synergy, or something like it. Like a triquetra.
What do the trio of Deathly Hallow do when working in concert?