In my game, I am using the "withered heart" as a plot device, not a magic item. I am treating it like a reliquary where the necromancer's spirit resides after his body was destroyed; it is pure spirit now and cannot reconstruct a body like a lich. I removed the side effect of instant death and, by attuning to it, you can telepathy communicate with the spirit trapped within; no need for "Speak with Dead."
The Arcane Brotherhood desperately wants to recover the withered heart so they can receive a de-briefing from the spirit of their agent that successfully infiltrated Dweomercore before he was killed by Halaster; though they do not know yet
how he died. One of the PCs wants to join the Arcane Brotherhood so, to earn their favor and willingness to accept him as a recruit, is attempting to recover the heart.
The Lords' Alliance has learned of this and recruited its own agent, a different PC, to recover the heart. They have received (and divined) ominous warnings of a mysterious arcane project being developed within Dweomercore that somehow involves the long-dead mad mage and believe that it is a credible and significant to the City of Waterdeep.
Note that only one PC, if any, will be able to complete their mission.
I feel that there are lots of little details within the adventure that can be expanded upon by a creative DM to become more meaningful but, as-written, are just "fluff." Seems like an opportunity for someone to create material for the
DMsGuild . . .