How do I handle active effects on a creature that then splits?

Question

I don't really see an area in the MM, PHB, or DMG that specifies what happens to effects that are active on a creature that then splits.

A case came up where my party was being attacked by an Ochre Jelly, our Hunter cast Hunter's Mark on it. I can't find anything specifying that Hunter's Mark isn't allowed on more than one creature, and I can't find anything stating that the Split reaction changes effects on a monster.

Is there a RAW solution to this issue, or a common way to handle this that is obvious?

Answer 1

The RAW

Hunter's Mark can be applied to more than one creature at a time but only with something like the Sorcerer's Twinned Spell Metamagic class feature.
Otherwise, Hunter's Mark requires concentration and you can't concentrate on more than one spell at a time (PHB pg.203) so you wouldn't normally be able to have it applied to a second creature.

As a strict reading of the Ochre Jelly's Split description and as Miniman points out, it splits into two new and separate Jellies so any effects on the original fade into the æther and aren't applied to either of the two new ones.

What you can do instead

As the DM you have the freedom to bend/break the rules as you see fit to make the game more entertaining and I'd say in a situation like this it'd be a fair call to do so.

Assuming the PC can maintain concentration, allowing Hunter's Mark to still be applied to only one Jelly when it splits isn't really going to break anything since you can change quarries when you drop your current one to 0HP anyway. You could even add a Constitution saving throw to maintain concentration when the split occurs to simulate the change in the creature having an adverse effect on the spell. Doing so brings this ruling closer to RAW and gives the player a chance at not wasting resources because of strict adherence of the rules.
This may also help the player feel less cheated if they only recently cast Hunter's Mark before the split and if the Jelly has high enough HP left after the split to make it's continuation seem useful.

Answer 2

Your Hunter's Mark would no longer be on either creature. If you look at the Split ability, it uses the following wording:

Split. When a jelly that is Medium or larger is subjected 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.

It seems pretty clear from the use of "new jellies" and "original jelly" that these are separate, freshly created creatures. Therefore, effects like your Hunter's Mark aren't on either of them, because they only just came into existence.

Answer 3

The way I personally handle it is that once split, the two halves both have their own stats and healthbars but are still the same 'creature'. If the ranger marked the creature as their quarry before the creature got split, the ranger gets bonus damage to both halves. There aren't any written rules on not being able to mark more than one creature as your quarry, though keep in mind it's a limited action anyway (since it's not a cantrip).

Since there are no written rules about having multiple quarries, It's up to the DM to decide if the ranger can have more than one quarry at the time. The way I play it is that rangers only can pick one prey at a time as their quarry. This way there's more of an urge to kill the targeted enemy, since the Ranger gets a free bonus action to select a new quarry once the initial quarry is brought to 0 hit points.