The metallic slivers are used in place as the token

Slivers

          Slivers are creatures that share a hive mind, which allows them to share their abilities with other nearby slivers.

            Slivers are depicted with an armored vertebrate body, a long, bifurcated whip-like tail, one arm with a single talon for a hand, and a head with an armored crest. Variations on this appearance exist in accordance with the abilities each has to offer to the hive.

Storyline

The first slivers shown in the storyline lived in Rath. It's unknown from where they originally came. Volrath is said to have genetically modified them, but to what extent is also unknown. He attempted to create artificial slivers, but these were a poor approximation of the real thing and had no innate abilities to share with the hive.
Volrath used the hive to guard the Legacy and to attack Weatherlight and her crew on their quest to retake those artifacts and their captain, Sisay. Thanks to Hanna, who realized how the slivers' hive mind worked, the crew defeated them in the Furnace of Rath and Karn was able to convince the Sliver Queen to release the Legacy to him, arguing that the artifacts were as much a part of him as the slivers were a part of her.
During the Rathi Overlay the slivers were transported to Urborg. Most of them were killed when they materialized in the heart of a volcano, and all the others were killed in the subsequent battles of the Invasion.
One hundred years later, wizards involved in the Riptide Project on Otaria discovered sliver fossils on their journeys and planned to bring them back to life in order to study them. The Riptide wizards did not know the importance of the Sliver Queen, however, and didn't recreate her. Without her, the slivers went rampant and overran the island housing the project.
At the same time the Mirari was causing havoc all over Otaria. The magical waves it produced accelerated the slivers' growth, but it also lured them to the mainland since they confused its call with that of their deceased queen. Most of the slivers were therefore present at the battle at Averru and were killed in the magical explosion that created Karona. A small group of slivers survived, however, fused into the ultimate manifestation of the hive mind, the Sliver Overlord.
Slivers that survived the apocalypse caused by Karona's destruction and a few breeds that were "timeshifted" into the present by the violent upheavals of the time stream were one of the few creatures to flourish on the dying plane of Dominaria. As other species fought for survival in a world almost completely drained of mana, the slivers continued to multiply and mutate into new forms. Some of these new breeds mimicked traits of other life forms in the multiverse; the vampiric sliver fed upon the life essence of its prey and was strengthened in return, much like the dreaded Sengir clan and their ilk. The gemhide sliver adopted an ability similar to that of the fabled Birds of Paradise, creating mana of any color whenever it wished. Basal slivers possessed the innate drive to sacrifice themselves to provide for their nestmates, similar to the way the master breeders of the Ebon Hand used Basal Thrulls for their dark rituals. Another new strain present during this time was the shadow sliver, trapped between two worlds when the nest within the bowels of Volrath's Stronghold was overlaid on Urborg. Presumably, this breed was a terror to the Rathi Tribes who were also caught in the overlay in their home, the City of Traitors. Without the leadership of their Queen, these sliver swarms raged across the plane, largely unstoppable when gathered in great enough numbers. The destruction caused by the slivers was not necessarily malevolent, the hive-minded creatures only sought a new queen, new leadership to guide their numbers.
For a time, the planeswalkers Freyalise and Lord Windgrace maintained some control over the slivers in their respective realms. Once the Weaver King discovered the simplicity of their hivemind, he quickly took control of them for his own purposes and used them to wage war on their former masters. Since his destruction, the slivers are presumably once again without leadership, though it's indicated that the slivers' hivemind is slowly becoming sentient and self-aware, negating further need for a Queen.
The Hivestone, a strange artifact discovered within the ruins of the Stronghold, was later found to have been used by the various evincars of Rath's past to control the seething sliver hive and its Queen. Upon its rediscovery however, the device became a curse to its wielder, bending its possessor's will to the will of the swarm.
Slivers are implied to survive well into the future, taking on even more new and bizarre abilities.