I'm running Tomb of Annihilation and
the party has recently gone to Nangalore and they encountered Zalkore,
a medusa. They killed her and one of the players wanted to take her head and I allowed it.
Later on, they had a random encounter and the same player said he wanted to use the head of the medusa to try to turn the goblins to stone.
Are there any rules on this or is this a DM choice?
Best Answer
Generally, no
There is no ruling of any kind in any official 5E material as far as I'm aware.
With what is given in the description of Petrifying Gaze, I'd rule that the head must be preserved and/or magically prepared in a way that would make it more than just a slowly rotting head.
Emphasis mine
Personally, I'd interpret 'not being incapacitated' as a state of being that the head of a dead medusa can't retain on its own.
Regarding her ability to petrify I'd consider it a byproduct of her curse that manifests whilst intently staring at a creature. Regarding arcane preservation/preparation I see no reason why a player shouldn't be able to have the head focus its gaze on a location/creature the wielder wills it to.
To prevent the petrification from being too powerful a tool you could limit the amounts of times the head can be made to focus, lower the DC or even prepend the DC with a mock ranged attack, as focussing someone else's eyes manually upon another creature's eyes.
As suggested in the comments by @Pixelmaster I'll add a brief chunk regarding the itemization of the head. I'll be referring to the
Flesh To Stone, a 6th level spell, would be the closest equivalent to Petrifying Gaze for players. The closest RAW item dealing a comparable status would be the rare Wand of Paralysis (Dmg P211), which has 7 charges and paralyzes a single Creature for one minute. If you wanted to you could base the stats of the head on the wand like so:
Note that the number of charges and their expenditure are deliberately vague. While a Wand of Flesh to Stone would be a rare item like the Wand of Paralysis their respective Spell versions are quite far apart. As far as I see the Wand of Paralysis is based on Hold Person, a 2nd Level spell, only the want targets Con instead of Wis. Giving a higher level spell less, or maybe even just one use per day may be the way to go.
Another thing to keep in mind would be that preparing the head for use should be a journey of its own, a little sidequest to discover some lost knowledge the character needs to force a spark of life back into the eyes of the medusa. You could also have the character roll several times through the creation of the item and have the Charges and DC based on how well they do.