The Medallion of Takar is an artifact used to trap the Viking ghosts of Takar. It has a blue gem in the middle of it which is surrounded by four stone gray axis into a round cross symbol. The axes seem to be metallic while the light blue within the crosses and circle seem to be stone-like.
It is first revealed in It Takes a Pillage as one half of a whole, possibly to avoid the ghosts ever being able to return fully. The cursed viking souls inhabited that first part of the medallion, possibly being trapped in its' gem. When their ghosts are released they immediately inhabit host bodies in the form of June's classmates and friends.
The curse of the medallion
From the onset, it appears that the Wiking ghosts of Takar are cursed to inhabit the medallion for an eternity. For what reason they were trapped, it is unclear. The first part is found in Orchid Bay retirement home and the second is found on the island of Takar, in the O letter of a jewelry store's sign called "Cursed Medallions & Things".
When June ends up having to hang onto the ledge of the store, holding onto Ray Ray Lee, who is holding onto Jody, posssessed by the leader of the viking ghosts, she demands for a way to break the curse. Monroe cites from his book of Te Xuan Ze, the medallion of Takar was created in 839, by Tasmanian warlodge The Wardoff of Blunderburg. Here Monroe is cut off by Ray Ray to shorten the story, so he doesn't have to drop Jody who is still possessed by the leader of the viking ghosts.
According to what the lore book details, the curse can be broken by thrusting both halves of the medallion itself "into the fiery depths of mount Kiln on the second moon of the harvest."
Because Juniper Lee does not have enough time to do it properly, in order to save her friend, she simply crushes it after asking Monroe if that is possible as well.
The viking spirits are thrust out of the bodies of the possessed children and all of them swirl into nothingness while ghostly wailing in pain. It is unknown what happens to them afterwards, but one ghost similar to them was seen as a background character in Season 3's Sealed with a Fist, so it is possible that if a ghost did not inhabit a body at the moment of the medallion being crushed, they could have kept existing as a body-less spirit.