Shandalar is a rogue plane, drifting through the multiverse instead of retaining a steady position or course. It is a relatively small plane and incredibly rich in mana. The magical energy is so prevalent that it all is sentient, and the common people use minor spells as an everyday convenience, and even sometimes use magic items to summon creatures to do their bidding.

Those who examine the amulet too closely have been known to become part of its ornate carvings.

In MtG, the idea of summoning involves the creation of a faux entity based on the concept of that creature, which is pulled from the aether. These summoned creatures have no will of their own and vanish when no longer needed. This concept is described in «The Eternal Ice» by Lim-Dûl to Jodah. Though Jodah is not in fact summoned, as Lim-Dûl suggests, the explanation is still valid. The short story «Loran’s Smile» further elaborates upon this summoning model and shows it firsthand. Feldon learns how to summon from a scholar who says to study him for two weeks; after the two weeks have passed, Feldon is able to understand what makes this scholar a scholar and is able to summon this concept of a scholar as a creature.

The summoning of unique, legendary individuals is similar to this, but based on the concept of a specific individual and not simply off of a group. «Loran’s Smile» also elaborates on this distinction: Feldon’s summoned copy of his mentor only stems from the concept of a scholar, resulting in a creature with various physical differences from the original, but he is able to summon a perfect copy of his late lover Loran by recalling his memories with her.

Planeswalkers can only summon creatures they’ve personally interacted with so that obviously limits them to creatures from planes they’ve visited. Ordinary wizards can therefore only summon creatures from their own plane. These summoned creatures leave corpses when they are killed, though they can also simply fade away when their summoners lets the summoning spell lapse.

In Dungeons & Dragons

Summoning has taken different approaches in D&D, depending on the edition. Fifth’s summoning spells were recently introduced in «Tasha’s Cauldron of Everything» in individual entries with different levels grouping similar creatures by type (undead, fey, etc.). This magic item allows its user to collect certain creatures to summon them later.

Quicksilver Amulet
Wondrous item; rare

You can expend a spell slot of the indicated level to imprint a creature that just died into the Quicksilver Amulet, its silhouette forming in the carving of the amulet, which gains the same number of charges as the spell level used. The amulet can have up to nine charges, which it gains back at dawn if used. You can «blank» an imprinted creature during a short rest.

While holding the amulet in one hand, you can use as many charges as the correspondent level to summon the creature imprinted on it, just as the mentioned spell.

1. No spell available
2. Summon Beast, Summon Fey
3. Summon Shadowspan, Summon Undead
4. Summon Aberration, Summon Construct, Summon Elemental
5. Summon Celestial
6. Summon Fiend

A summoned creature gets not only the powers and features of the creatures indicated in each spell, but also their original ones (with the DM permission, of course).

In Eberron

If it exists in D&D, it also exists in Eberron. The Quicksilver Amulet can be a magic item designed by Oalian as a reward for their most valiant druids to preserve the souls of beasts, or maybe a item which only members of House Vadalis can activate with their dragonmark.

How are you planning to incorporate the Quicksilver Amulet in your campaign? Are you willing to take the risks of imprisoning a hostile creature within it with hopes of using it against your enemies later?