Promethium Lockbox

A Promethium Lockbox is a rare box item dropped randomly by any enemies or players defeated by the player in any instance once they reach level 10.

With each lockbox made from Depleted Promethium; a metal invented and patented by Steve Dayton of Dayton Industries, the lockbox can only be opened through the use of specialized "keys" that contain a chemical known as Volatile Promethium that can eat through the lock.

Starting April 3rd, 2012, once opened the player will be rewarded with a unique consumable item or armor style piece that can only be found within Lockboxes. Due to the unstable nature of Promethium, the player will have 30 days to open a Lockbox before it degrades beyond use. The box's status can be checked by checking the tooltip on the Lockbox in the inventory.

Game update 26 reorganized the content to be found in lockboxes completely. Now style items and exclusive base items will be contained.

Additionally a Mark of Victory can be found in each box.

Since Hotfix 22/07/15 Promethium Lockboxes feature a Loot Picker, as well as the first time faction-only Vender styles can be obtained cross faction.

Additional Styles
Additionally accound bound single style items for the following costume style sets may be contained:

Aegis of Azarath, Aeronaut, Adaptive Android, Angelic, Antifreeze, Archangel, Avatar Bombardier, Avatar Infiltrator, Biomech, Contemporary Tech, Dark Specter Batsuit, Druid, Elemental Android, Fate's Faith, Frozen Fury, Gotham Knight, Greco-Roman, Hera's Strength, Hijacked Servitor, HIVE Defender, House of El Warsuit, Jester, Joker's Punchline, Kabuki, Kryptonian Commander, Kryptonian Medico, LexCorp Salvation, Malevolent Knight, Mayan, Metallo's Maw, Necromancer, New Genesis (Style), Nth-Metal Battlesuit, Oolong, Paramilitary, Plant, Psycho, Punchline, Raptor Tech, Retro Tech, Shaman, Schoolyard, Speed-Force Spectrum, STEELsuit MK-1, Sunstone Bulwark, Sunstone Edge, Tech Ninja, Vengeance, Vengeance of Hecate, Vengeful Surgeon, Viking, Virtuous Knight, Winged Fury, Wizardly, Zonewalker

Former Items
Game update 26 brought a complete revamp of the contents of Promethium Lockboxes. Formerly the boxes contained legendary low level gear and legendary tier 1 to tier 4 rings and necklaces.

For an overview of those please see Promethium Lockbox - Former Items.

Trivia

 * While players with Legendary Membership have an unlimited supply of keys to open Lockboxes, players with Free or Premium Access will have to purchase a Promethium Key from the Marketplace from April 10th, 2012, onwards to open them. A Promethium Key will cost $2.49/250 Station Cash. A bundle of 5 will cost $9.99/1000 Station Cash.
 * When Promethium is alloyed with Titanium and Vanadium, it forms a near-invulnerable metal.
 * The bionic and cybernetic components of Cyborg are made out of Depleted Promethium, Deathstroke's mesh armor, sword and staff are also made of Depleted Promethium.
 * Volatile Promethium is also capable of generating near-limitless amounts of energy and can be used as a power source for many gadgets. Volatile promethium is also a dangerous mutagen that can be used to mutate living beings or trigger their metagene.
 * As revealed by Mepps, it is not really possible to "farm" lockboxes. "Your chances of finding them are based on time, not exactly on the amount of kills you get. You'll get just as many lockboxes going about your business and playing the game normally as you would going out and slaughtering everything in sight." Until game update 26 there was also a cap (15) for how many Lockboxes a character can find, that reset every 30 days.
 * As of July 2015 a search in the Daybreak Games Census Data reveals 12 different item IDs for Promethium Lockboxes.