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.
Additionally account bound single style items for the following costume style sets may be contained:
Adaptive Android, Aegis of Azarath, Aegis of Eternity, Aerial Defender, Aeronaut, Alien Tech, Angelic, Antifreeze, Archangel, Archer, Avatar Bombardier, Avatar Infiltrator, Barbarian, Biker, Biomech, Bulwark of Madness, Contemporary Tech, Dark Specter Batsuit, Demonic, Divine, Dresden 7, Druid, Egyptian Sorcerer, Elemental Android, Fate's Faith, Formal, Fourth World, Frozen Fury, Gadgeteer, Gotham Knight, Greco-Roman, Heartshard, Hera's Strength, Hijacked Servitor, HIVE Defender, House of El Warsuit, Insectoid, Jah Kir, Jester, Joker's Punchline, Kabuki, Kryptonian Commander, Kryptonian Medico, LexCorp Salvation, Malevolent Knight, Mayan, Medieval, Mercenary's Malice, Metallo's Maw, Necromancer, New Genesis (Style), Nth-Metal Battlesuit, Oolong, Paramilitary, Phantom Zone Reaver, Plant, Psycho, Punchline, Raptor Infiltrator, Raptor Tech, Rapture, Retro Tech, Schoolyard, Seraph, Shaman, Shielded Robot, Shroud of Anubis, Snake, Speed-Force Spectrum, STAR Ex, Steampunk, STEELsuit MK-1, Sunstone Bulwark, Sunstone Edge, Talon Lord, Tech Ninja, Vengeance, Vengeance of Hecate, Vengeful Surgeon, Viking, Virtuous Knight, Voodoo, Winged Fury, Wizardly, Zonewalker
For an overview of those please see Promethium Lockbox - Former Items.