Shard of Death

A crystallized manifestation of death itself. It can be created under rare circumstances when a creature passes away surrounded by strong necrotic energy. The energy somehow collapses into a singularity coinciding with the creature's soul. Most of the very few documentations of such an event mention the presence of a gate of death nearby, a magical passage that connects a realm to one of the realms of death. The shard appears as either a clear, icy-blue, or bright dark-green crystal with necrotic energy spreading around in it like ink spreading in water.
When near a creature that has died within the last hour (excluding undead and constructs) or when near an undead, the shard stirs in death, becoming unclouded and revealing the essence of the souls within. In this state, a creature attuned to the shard may use its bonus action to gaze into it and draw from it an aspect of the souls infused into the shard. A creature gazing into the shard rolls on the shard's spell table to draw out a specific spell; after doing so, it makes a DC 12 Intelligence check. On a success the creature memorizes the drawn-out essence, allowing the spell to be drawn from the crystal without gazing into it. On a failure, the spell remains accessible for 20 seconds after gazing into the shard. If the creature rolls a nat 20 on the Intelligence check, they may learn the spell after casting it once through the shard within the next 20 seconds.
A creature may gaze into the shard safely once per long rest. Gazing into the shard repeatedly will cause the creature's soul to be drawn in, forcing it to suffer two hit die worth of necrotic damage for each gaze after the first one since its last long rest; this damage cannot be reduced in any way.
An attuned creature may cast a drawn-out or memorized spell through the shard, using the shard as the casting focus for that spell. Casting a spell through the shard does not consume spell lots, instead, the caster rolls a number of its hit die equal to the spell's level and takes this damage as necrotic damage. This damage cannot be reduced in any way, however, it cannot bring the caster below 1 hp. When the shard stirs in death, the caster may subtract a number of hit dies rolled equal to the CR divided by 3 (rounded up) of the creature to which the shard responds, to a minimum of 1 die to be rolled. If this creature has no CR, subtract a number of dies equal to half the spellslot level used to summon it (rounded up; if applicable) or otherwise subtract 1 die. This effect can only be applied once per creature unless that creature is revived. If the shard is affected by multiple creatures, the caster may choose for which creature it subsctracts a number of dies. Only one creature may be chosen at a time. The shard cannot be used to upcast spells.