A tantalus is a nonmagical piece of furniture, a liquor-stand that uses wooden or metal rods to keep the bottles and glasses in the stand from moving about. Tenser's tantalus is a mobile, magical version of the same sort of device, designed to transport magical potions or other fragile items, keeping them safe and within easy reach for the mage's use.
Although there is no evidence that this magical item was created (or even used) by Tenser, the famed wizard of Greyhawk, the reasons its origin was erroneously attributed to him are obvious; Tenser's tantalus is a flat metal disc, 3' in diameter, which floats at a constant 3' above the surface of the ground (under most circumstances) and at the same 3' distance from its user unless specifically commanded otherwise. But while it shares much in common with the Tenser's floating disc spell, it also has a number of significant differences, the primary one being that Tenser's tantalus is a permanent magical item, not a temporary magical spell. The disc's upper surface emits a soft glow extending for about a foot above the metal. Any objects placed entirely within this field are subject to a powerful, quasi-magnetic force that holds any nonliving matter (including once living materials such as paper, leather, or a small dead body, as well as completely inanimate matter like glass, metal, or crystal) firmly to the surface of the disc, but that has no effect on living flesh, even creatures small enough to fit within the field's confines. No matter what pressures are exerted against it, the tantalus will protect objects within its field from the effects of violent motions and other external shocks unless those shocks are powerful enough to destroy the metal disc. For this purpose, treat the disc as a metal shield, using Table 29: Item Saving Throws in the DMG. Since the quasi-magnetic force doesn't affect objects only partially within its area of effect, bottles taller than the 1' field cannot be held by a Tenser's tantalus unless laid on their sides. Bottles can be stacked, but that is difficult. Reaching through the glowing field and grabbing an object on the disc frees the object-when an object is grabbed, the field reacts as if the object is physically connected to the arm and thus as if it extends beyond the edge of the field.
Because objects only partially within the field aren't affected, a thrown spear can wreak havoc on the disc's load-a spear is long enough that part of it is always outside of the field. On the other hand, a sling bullet hurled at the bottles cannot break them because the field affects it as soon as the bullet is fully within its boundaries, leaving the bullet suspended in midair, just inside the edge of the field. For maximum protection, potions should be placed as close to the center of the Tenser's tantalus as possible, protecting them from weapons shorter than the 1' radius of the disc. If the disc is fully loaded, items close to the edge can be easily affected by short weapons such as hand axes or daggers.
A Tenser's tantalus can hold up to 20 pounds of bottles and contents (the average potion in a glass bottle weighs about half a pound), and will smoothly accelerate and decelerate to safely, following its owner at any speed up to 18, including climbing stairs. If the user moves beyond the 50' effective range of command, the tantalus ceases all movement and hovers in place until either its user returns or another wizard takes control of it.
A Tenser's tantalus will follow its owner if he or she falls into a pit, but because of the owner's rapid acceleration, the wizard will soon out-strip the disc's power to follow. In a pit less than 75' deep, the disc gently descends to the bottom a few seconds after its owner. If the pit is deeper, the owner will eventually accelerate out of the 50' range, meaning that the disc will stop in midair some 50' down from the top of the pit, waiting for someone to come within range and take command.
Because such an obvious display of treasures increases the possibility of theft, many Tenser's tantaluses are equipped with a permanent alarm spell with a volume loud enough to be heard from 50' away, but no farther. The alarm sounds when anything enters the field without speaking the command word that deactivates the alarm. Semi-solid manipulators, such as unseen servants, telekinesis, or Bigby's hand spells, air elementals, or gusts of wind are all too tenuous to reach into the field and take or destroy items, but all of these will activate the alarm.
A thief could attempt to steal the entire disc away from its owner. This requires a Strength check on 4d6 to break the disc's attraction to its owner. If the thief takes the disc more than 50' from its owner (and remember, the alarm will sound unless the thief manages to touch only the bottom and rim of the disc), it is free to be commanded by another wizard.