The off-hand zephyrium warhammer is an off-hand weapon used in Dungeoneering. It may be produced using the Smithing skill or obtained as a monster drop.
Even though this weapon is available to F2P, it has not been added to the drop table, causing F2P players to be unable to obtain it. The only way to have one on a F2P server is to bind it on a members server and bringing it to a free world.