The Reins of Unity (Japanese: キズナのタヅナReins of Bond) is a Key Item introduced in Generation VIII. It can be used on Calyrex to fuse it with Glastrier or Spectrier into Ice Rider Calyrex or Shadow Rider Calyrex or to separate Ice Rider Calyrex or Shadow Rider Calyrex into its constituent Pokémon.
The Reins of Unity can only fuse together one Calyrex and Glastrier or Spectrier at a time. In order to fuse a Calyrex with a different Glastrier or Spectrier, any existing fusion must be undone.
Reins that people presented to the king. They enable those who are connected to communicate with each other. The reins separate Calyrex and its beloved steed.