Hi there,
within the rulebook on the pages 9 and 10 the spells are explained. As soon as one plays a spell on his active turn, he may put a die he rolled during his current adventure on this cast spell. Since there is no telling you otherwise you may put the red die with the Wildcard-symbol on such a spell, when you play a unique item allowing you to put the red die into your dice-pool.
The cast spell is part of the play area (pg 9).
A die remains on the spell card until it is used to complete a task (pg. 9).
The die remains on the spell card until it is being used - even on a different player's turn. (pg 10)
Just for clarification:
This would mean that player A uses a unique item to roll the red die; player A then plays a spell in order to put the Wildcard die result on this spell. And any player B, C, D or even A in another, later turn may use this spell-bound red die with the Wildcard as a joker, right?
Or does this rule about the red die interfere:
In any event, the die is removed from the dice pool at the end of the active player's turn. (pg 9)
All the best!
Mad