SICK from thinking about the symbolism of the rune.
Which rune Viktor gives baby Jayce is the only thing we see as a catalyst for change across timelines — it's the butterfly flapping its wings.
The rune switches hands only twice more after that:
Viktor giving it back to Jayce, representing the start of their partnership concerning Hextech,
and Jayce giving it to Viktor in the astral plane, helping him put an end to the arcane apocalypse.
But there's one more important similarity about these moments — they're moments when Viktor and Jayce save each other.
Mage Viktor saves Ximena and baby Jayce, then gives him the rune.
Viktor stops Jayce's attempt, then hands him back his bracelet.
Jayce stops Viktor from losing his humanity, taking the rune from his wrist and together, they use it to save the world.
The rune isn't just about possibilities. It's a representation of their lives.
And they only ever put it in each other's hands.