[Edi]

Slave collars, hearts, Flesh Ward, Twisting Thorns and all eye items are non-findable, as are Lifelong Protection and Soul Contract. They can never be found by ANY commander if the owner is killed, they are gone when that happens.

Knife of the Damned is an annoying one that can be used for this, but there are far better uses for the astral and nature gems that would otherwise go to its making, especially in any sort of quantity.

In case someone wonders, the reason for why Lifelong Protection and Soul Contract are non-findable is found in the item descriptions. It is a contract written in blood between the Infernal Powers and the recipient and once the recipient dies, the Infernal Powers claim his soul and the contract ends. At that point it's just a scrap of parchment, though it could just as well go up in flames upon termination (for dramatic effect).