We usually follow this homebrew guide for magical item prices, and my players have recently played through the Lost Laboratory of Kwalish adventure. In it, they obtained a cursed longsword, a Polymorph Blade. It casts a Polymorph on the target, randomly ranging from a TRex to a rabbit.
When you attack a creature with this magic weapon and roll a 20 on the attack roll, the creature must make a DC 15 Wisdom saving throw in addition to suffering the attack's normal effects. On a failed save, the creature also suffers the effects of a polymorph spell. Roll a d20 and consult the following table to determine the form the target creature is transformed into.
The blade is cursed.
This weapon is cursed, and becoming attuned to it extends the curse to you. Until the curse is broken with a remove curse spell or similar magic, you are unwilling to part with the weapon. Whenever you attack a creature with this weapon and roll a 1 on the attack roll, you suffer the effect of a polymorph spell for 1 hour, rolling on the table to determine your new form.
They have now decided to sell it (we follow a homebrew world where some places have magical item markets), and I was wondering what would be an adequate value for it. It is very rare, cursed (my players don't really care about that), and does not look too powerful.
I would value the Polymorph Blade around 5000g (a bit over a +2 longsword). My reasoning is that Polymorphing the enemy you're attacking is situational (either there are other enemies to attack or it is not worth it) and can backfire (suddenly facing 136HP of TRex instead of a random minion). On the other hand, it can take the BBEG out of the fight while its minions are overrun, and can also burn legendary resistances. The effect only occurs on a natural-20, so it should be rare. I'd like to have some input from the community on my value estimation.