Chronology

Death: At Death's Door

Taking place just after Lucifer abdicates his throne, Death realizes how tedious chasing after billions of vagabond souls alone would be and thereupon enlists her sisters Despair and Delirium to aid her. In doing so, she creates two ankhs, each with similar powers to her own (e.g. act as a radar for the dead, absorb and teleport souls, etc.).

Death: The High Cost of Living

In her first spinoff from The Sandman, Death creates a mortal avatar of herself, Didi, to live amongst humanity for a day. Sometime later, she gets mugged for her ankh by an eremite who exclaims the charm holds all the secrets of life and death. Didi, unworried, informs him he doesn't have her and thus has nothing.

An hour or so before her time is up, Didi buys a new ankh, the one thing she pays for, implying the old one was useless in the wrong hands and stating it the most important object in the universe.

The Girl Who Would Be Death

The premise of the story is that one of Death's ankhs falls into mortal hands (i.e., the wrong hands), implied the same pendant that was stolen in the High Cost of Living. Upon making contact with it for the first time, Plath, who sought out the ankh, died, but remained in the flesh. Unlike the eremite, she retained the knowledge of how to use the sigil, warranting Death's attention, who was already attempting to recover her possession.

Playing by the rules for whatever reason, she sends an envoy to retrieve her stolen property. Plath, however, uses the power of the ankh to deter any opposition, ultimately becoming a medium, opening a dimensional rift and destroying the barrier between life and death. Telepathically communicating with Plath's lover, Evangeline, Death instructs her how to become a conduit for her power, allowing her to usurp Plath and steal the ankh; whereupon she uses it to vaporize a death worshiper. Then finally, Death teleports her to safety and incinerates Plath along with the pendant.