Possibly a 1 turn silence would be more thematically appropriate.
The reason to pay Charon was to ease the passage of the soul into the underworld; if Charon wasn't paid, the soul would be forced to wander for many years along the bottom of the river Styx. I am not sure how a relic makes sense, but it would make sense for the penalty for not paying to be some sort of delay.
yah so the relic was a suggestion from the pre smithy. if you read the description of the card relic it reads as follows: The remains of what was once a valued creature or artifact. Probably still worth a few coins.
the reason relic was suggested is that it would serve as a symbol of your dead creature's lost spirit wandering back into your hand. making a new card such as (lost soul) is more creative but i didnt see a reason to make a new card just as a placeholder card if relic fit.
and placing a relic in the hand does serve as a delay (if their hand is full) by delaying draws before you can play this useless card.
hope that helps the card make more sense.