The Grail shard belonging to the player character’s faction.
The lore mechanic I’m currently mulling over is the idea that the player character can safely warp to the past because they have a Grail shard of their own.
Either they’ll have a Grail shard implanted as a cyborg part, or they’ll be accompanied by a Grail fairy (Navi).
The point is that Holy Grails are drawn to each other.