This device can only be created by Ultimate-level Immortals: Thanatos is the only Immortal known to have used it.
The sheath can imprison an Immortal victim. It draws on the victim's own power to trap the prisoner in a pocket dimension, accessible only through the Time Stream. Whenever the victim tries to exert power, the sheath feeds on the power and shifts the victim back a few seconds in time to the point before he or she decided to use the power. Thus, the victim remains perpetually on the verge of action, without acting.
To an onlooker in the Time Stream, the shift backward in time makes the sheath appear to recede. All objects that move forward in time appear to approach the observer.
Thanatos trapped the Immortal Asterius in this sheath shortly after the Other Immortals were lured back to the headwaters of the Emerald River. Asterius, though, cleverly foiled some of the sheath's restrictions. By reciting certain spells backwards. Asterius provoked the sheath's movement back in time-and that, in turn, repeated the recitation of the spell forward, triggering its effects.