Every Friday - we call it Xur Day - a special non-player vendor character arrives in the Tower, Destiny's social space. The mysterious merchant, eyes burning white, tendrils reaching out from within a face cloaked in shadow, sells Exotic weapons and armour, but there's no way to predict which ones. Xur, Agent of the Nine, is a law unto himself.
Or is he?
Exotic class weapons and armour are the best in the game, but getting your hands on them is not an exact science. Complete a Nightfall mission and you have a good chance to get one. Defeat a boss in one of the game's two Raids and you have an okay chance to get one. Complete a match in the Crucible player versus player mode and you have a slim chance of getting one. And then there are the various drop rates for each Exotic item to contend with. Not all Exotics, it seems, are created equally.
Take Gjallarhorn, for example. This Exotic-class rocket launcher is considered the best in the game. Some say it's essential to high level success. I've played Destiny since its launch in September 2014 and still haven't got it. Xur sold it the second week after launch. Back then, I was but a Destiny child and failed to recognise the significance of the event. Xur hasn't sold it since. Did I miss my chance?
When Xur turns up in the Tower on Friday morning he turns heads. What's Xur selling this week, the collective Destiny community wonders. There is a rush to find out. Video game news websites publish articles about it. Videos are frantically uploaded to YouTube. The popular Destiny sub-Reddit updates with the latest "Xur Megathread", chronicling the Exotic gear, curios and upgrades he's selling that week. "Who is Xur?" the Megathread asks. "TL;DR: He's the Santa Claus of Destiny and every weekend is Christmas."
Bungie tells me it sees a spike of activity within the game when Xur arrives in the Tower, usually 2am Pacific in North America, 10am UK time. There are many who set their alarms so they can check Xur's inventory the minute he materialises. And then, on Sunday morning, he vanishes into wherever in the galaxy an Agent of the Nine likes to call home.
This community-wide anticipation for his arrival is entirely manufactured on Bungie's part.
"We definitely wanted that rhythm," user research lead John Hopson tells me.
"The, it's Xur day! It's time to log in. Maybe I haven't been playing this week, but, oh, let's see what he has! There's that little bit of a nudge to log back in, see what's happening and maybe get into a game and maybe go on playing with your friends. That was always part of the intention."
An intention wonderfully executed. I don't have all of the Exotic weapons in the game, but I want them. I suppose Destiny is my Pokémon, the "gotta catch 'em all" effect in full flow. I still need Gjallarhorn, I still need Red Death and I still need MIDA Multi-Tool. Xur is my ticket to the set, the full Vault, the complete sticker album. So each week I play Destiny to get as many Strange Coins as possible just in case Xur has something I want. Each week I wait in hope Xur will pack Gjallarhorn, Strange Coins at the ready. I'll do anything, Xur, just pack Gjallarhorn. Anything...
"It was always part of the intention to have pity Exotics for people who, they didn't get the drop but eventually they'll be able to buy it off Xur and have that little catch-up mechanic that you will eventually get the gun you want," Hopson explains.
"You may have to wait until it comes up in rotation, but you'll eventually get your gun."
On the Destiny sub-Reddit there is a superstar Japanese player called "megamanexe4", or, simply to the community, "megaman". His name is apt. Megaman has spent months data-mining Destiny to reveal unannounced information about missions, loot and even future expansions, and along the way he has accurately predicted Xur's inventory on multiple occasions. Players love megaman because he helps them work out whether to spend their heard-earned Strange Coins or save them because they know something they want is just around the corner.
Over the past six months there has been a game of cat and mouse played out between Bungie and Destiny data miners such as megaman. Only two months ago, players who were trying to get the skinny on Xur's upcoming wares discovered Bungie had broadcast every item in Xur's planned inventory as No Land Beyond, considered by the Destiny community to be the worst Exotic weapon in the game. It was an obvious - but good-natured - troll.
Now, it seems, Bungie has the upper hand, and megaman has retired.
"Sorry. We can not use data-mining and glitch," megaman wrote in a post on Reddit. "Bungie has changed the Destiny system."
The suggestion is that Destiny now uses a new version of Xur that Bungie controls manually, and that the developer is forcing him to sell the Exotics it wants him to sell. Bungie has so far remained silent on this, and there's no confirmation either way, but according to megaman something has changed, and the upshot is he can no longer confidently predict Xur's inventory.
Still, using a list gathered from information mined from the old version of Xur, megaman's predictions continue to ring true. Only two weeks ago megaman predicted Xur would sell Thunderlord, the Exotic machine gun on 8th May. He was right.
"My last prophecy (there is no conclusive evidence)," megaman wrote in a post on Reddit on 8th May.
Megaman's prophecy was that this week's Nightfall would be the Omnigul Strike with the arc burn, lightswitch and juggler modifiers enabled. He was right.
Then, his final final prophecy (promise): "This is the last. Because Japanese community does not function, I am leave Destiny. Goodbye Guardian," he wrote on Reddit.
The final prediction: next week's Nightfall will be House of Wolves strike Belly of the Beast. We'll see.
The truth is, no-one outside of Bungie knows how, exactly, Xur works. There are many fan theories. There is much rumour and speculation. For months after Destiny's launch players accused Xur of hating Warlocks because he kept selling them the same Exotic gauntlets for weeks on end. So Warlock players launched "Operation: Murder Xur on Reddit (Hunters and Titans countered this with their own Operation: Protect Xur). Someone even went to the trouble of trying to work out the maths behind his apparent random inventory.
Back in February, Bungie community chief David "Deej" Dague addressed the issue in a post on Bungie.net. In it he said Xur's inventory "is governed by the same chaos that influences all the loot drops".
"We know what he's selling before he appears, even if we can't reach out and stock his shelves ourselves. We set him in motion, and his will is not his own.
"These conspiracy theories are symptoms of a burning desire. Gjallarhorn is the most popular ask that comes my way. I know you want it, even more than the collective requests for more vault space. As you can see, these are things that are beyond my control. I don't have one either."
Here's what we know: last week, Xur sold Thunderlord for the first time, which means he has now sold every Exotic weapon he is capable of selling. And by that we mean there are some Exotics Xur cannot sell. One is the Vex Mythoclast auto rifle, which is reserved as a drop from killing the final boss of the Vault of Glass raid on hard mode. He will not sell PlayStation timed exclusive Exotic weapons, such as Hawkmoon, either, presumably until Sony's exclusivity deal with Activision expires this autumn. And he will not sell Exotic weapons that are reserved as rewards for completing Exotic bounty missions.
Here's what I suspect: each Exotic has a percentage chance to appear in Xur's inventory. Some are more likely than others. Some, such as Gjallarhorn, have a tiny chance to appear. But there is a feeling among the Destiny community that recent events suggest Bungie has assumed direct control over Xur in a bid to ensure every Exotic item has been sold at least once ahead of the release of House of Wolves on 19th May.
Take Thunderlord, for example. Two weeks before the release of the expansion, Xur sells the gun for the first time. The week before that Hard Light went on sale - just the second time it's been available. Coincidence?
"The way people interact with randomness is so amazing to me," Hopson says. "The one that sticks out in my mind is from World of Warcraft and Onyxia. She was a boss that would do Deep Breath that would kill lots of people in the raid. People had all these theories about how you'd have to spread out and who she would target with this ability. The developers came out and said, 'no, it's random.' They have officially said, 'no, it's random when she Deep Breaths.' And I don't believe them. I got Deep Breathed so many times I don't believe them when they say it's random, and yet they're saying it. I'm sure they're truthful, but in my gut I don't believe them.
"There's this very strange thing about how people interact with randomness. And we don't believe it. We're not biologically built to believe things are really random."
In a roundabout way, I ask Hopson again: is Xur random? This time he stays silent, still smirking. A Bungie representative insists he is. So why not pull back the curtain? Bungie wants conspiracy theories. It wants debate and discussion, anger and speculation. It's all part of the cult of Xur, the myth, the legend. Remember that bit in The Wizard of Oz when the Wizard himself is revealed to be little more than a parlour trick? Be careful what you wish for.
Xur is my favourite character in Destiny, and it's not because he has great dialogue, or stars in emotionally-charged cutscenes, or is particularly cool. It's because of the impact he has on the game each and every Xur Day. Some will say Xur is little more than a conduit through which Bungie shapes the infamous Destiny grind and that I'm over-playing things somewhat. That's probably true. But for me Xur has more personality and panache than all of Destiny's disappointing story put together. He is a simple NPC vendor who shows up only at weekends but manages to outshines all the other characters in the game - the Queen, the Speaker, even the Postmaster can't compete. Xur is a bona fide celebrity NPC.
"Pay no attention to that man behind the curtain," I imagine Bungie saying as the Destiny community prods and pulls at the game. I'm happy to play along. Part of me wants everybody to leave Xur alone. Let the debate over his "free will" rage. It's fun.
"Xur is Xur," is all Hopson will say, with a wry smile. "I have no control over Xur."