Your Eternal Reward is radically different from the Spy's default knife in that it does not allow the Spy to disguise himself at will. However, the Spy gains the ability to perform silent backstabs.
Upon a successful backstab, the Spy will immediately assume the backstabbed target's identity and the corpse of the backstabbed player will disappear leaving no trace of the Spy's treachery.
Your Eternal Reward is part of the Saharan Spy set alongside the Familiar Fez and L'Etranger. When all three items are equipped, the volume of the sound that plays when the Spy decloaks is drastically reduced, negating the penalty of the Dead Ringer and rendering all other Spy watches completely silent.