The lvl 9 spell, shapechange, gives you all the physical attributes of whatever form you choose, including whatever natural weapon attacks and abilities come with that form, right? So a wizard shape changed to ochre jelly and fighting a blue dragon should split when hit by a claw attack or lightning breath as long as the wizard has at least 10 hp. What happens to the wizard? Are there now multiple wizards because there are multiple jellies and what happens if concentration is broken with a jelly or after the duration is ended?
What happens when a jelly that isn't a wizard splits? The Monster Manual (p 243) says, with my emphasis:
[...] it splits into two new jellies if it has at least 10 hit points.
As nothing describes these jellies ever combining to reform a single entity, it seems pretty clear that the new jellies are completely independent of each other. Splitting is jelly reproduction.
So, if the jelly in question happens to be a jelly-shaped wizard and it splits, the wizard has just reproduced. At that point, you have a wizard in the shape of a jelly sitting next to its offspring, a completely separate and autonomous jelly. The wizard's consciousness remains in a single jelly, while all of the jellies that split from it are independent creatures that remain after the shapechange spell ends. From the moment they split, the wizard and its jelly children lead separate fates.
To think of it another way, if a wizard shapechanges into an awakened plant would anyone expect the wizard's consciousness to be spread among all of the leaves it drops? Does a spider-shaped wizard have to make a concentration check if someone burns webbing it produced? For that matter, if a wizard-shaped wizard gets a haircut, would anything happen to that hair when the wizard changes shape?
I think logic pretty clearly dictates that once something ceases to be part of a creature, it also ceases to be part of a wizard shaped like such a creature.
A loathsome-limbed troll provides an exception that supports this argument. Several elements in the side-bar make it clear that the partitioned troll remains a single entity (MM, p 291). Severed limbs can be reattached to the main body. If they are not, the body regrows the part and the part dies. The troll clearly resides within the main body, not the arms, legs, or head. There is an implication of shared consciousness, as the troll is able to "see" whatever its head sees, regardless of the head's attachment to the body. Contrast this to how the division of jellies is presented. Jellies cannot recombine, they do not share information, and both jellies can potentially survive apart for an indefinite period of time.
If the wizard chooses to split, the wizard ceases to exist
First an ochre jelly's split reaction is part of the monsters statistics so the polymorphed wizard certainly has access to it. Split is a reaction so the wizard can choose to use it if they have their reaction available and it's triggered. But they probably don't want to.
When a jelly that is Medium or larger is subject to lightning or slashing damage, it splits into two new jellies if it has at least 10 hit points. Each new jelly has hit points equal to half the original jelly's, rounded down. New jellies are one size smaller than the original jelly.
What's important here is that the one jelly Splits and then there are two new jellies. While the ability doesn't spell out what happens to the original it does make it very clear that two, brand new, jellies are created. I see two interpretations here:
- Since Split doesn't say splitting destroys the original jelly there are now three jellys (1 original + 2 new jellys each with half the hp of the original and one size smaller). I don't think is the intention but I do think it would be a valid reading.
- Splitting destroys the original and now there are just the two new jellies. This seems correct to me and is how I've generally seen this type of monster run.
So what happens to our polymorphed wizard? In the first case he spawns two totally normal (though smaller) jellys. In the second case he vanishes in a puff of bad ideas and the party now has two newborn ochre jelly to deal with. I lean toward the second case.
This is an interesting case, but it doesn't work any differently to usual. The wizard can split into as many jellies as they want. When shapechange ends, the wizard will revert to their own form.
A troll with loathsome limbs would have the same problem. In the troll's case, the natural assumption would be that the wizard would revert when the main body died. However, this either leaves the wizard missing a limb or with a random extra lying around. The only way that makes sense is for the wizard to revert when all the bits of the alternate form die, and while the jelly is less clear it should work the same way.
The issue here is where the wizard will be if the jellies get separated. Personally, I would rule that the wizard would be at a point that had the minimum total distance from every remaining jelly.
I would also rule that the stress of controlling multiple bodies would give disadvantage on concentration checks, but that is entirely up to the GM.