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I have a PC with an OP weapon. it hits for large amounts of damage. Understandably he is rather attached to the sword after a number of years now so the horse may well have bolted.
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I don't know what OP stands for (P for Potent (item)?)
You stated that it is the "Whispering Sword" out of a companion.
If it is identical to the one out of "RM Classic Creature & Treasure",
it is interesting that this is balance breaking (it is stated as modest item ...)
If you want to make the PC to give it up and make it mysteriously
just increased the fumble rate by one:
"You rolled a '03' something changed: Make a fumble roll!"
Some time (playtime or realtime) later:
"You rolled a '04': Make a fumble roll!"
The end of the line? Maybe '08' maybe higher.
But why is the sword doing this?
(This presumes a kind of intelligence of the sword)
Maybe it will back to its creator or its original owner,
maybe it will away from this person with his battle anger (frenzy) or
maybe it is just time to change its wielder.
And the present user might never know.