Gjallarhorn is an Exotic Rocket Launcher in Destiny and widely considered the best weapon in the game (I haven't actually played recently but I assume nothing has yet surpassed it). In Destiny, an NPC named Xur appears in the central hub areas once a week, staying for Friday and Saturday only, selling a select number of Exotic weapons and armor that players can purchase with Strange Coins, a type of currency in Destiny. Exotic weapons can be very hard to come by in Destiny and are randomly earned from completing certain content, but since the reward system is all RNG-based, there is no way to work toward attaining these items in a guaranteed way. The only way to actually buy them is through Xur, but his inventory changes every week and offers only one weapon at a time, plus one armor piece for each class.

Xur was selling Gjallarhorn one week very soon after the game came out (I think the second weekend?), but at that point very few people had experienced the weapon and most of us assumed it was not worth its price since we had very limited amounts of Strange Coins at that point. Since then, it became very obvious that this rocket launcer was by far the most powerful weapon in PvE play and simply owning one was enough to convince some people to take you into their raid group. Everyone who declined to buy Gjallarhorn that one weekend has been kicking themselves ever since. Nearly a year later, Xur has not once sold Gjallarhorn again. Most dedicated players have been lucky enough to obtain the weapon now through other means, but still thousands of Destiny players wake up every Friday morning to check Xur's inventory, hoping against all odds to see the word "Gjallarhorn" in the list. This morning, they were finally vindicated as Xur is once again selling Gjallarhorn this week.