I am building an encounter for 6 or 7 PCs (mixed levels 4/5) with an Oni: it may turn into combat. Oni can be very difficult, even deadly, encounters for PCs at this level. What has me puzzled is how HP loss and the change shape feature work for an Oni.
Oni begins as a Large Giant, average 110 HP. What happens during the battle when the Oni changes shape into something else? My confusion comes from use of "polymorph" to describe the change shape ability. In the spell of that name the HP of the polymorphed form can be reduced without reducing the HP of the original form.
Combat Example
The party had done 76 points of damage (leaving the Oni with 34) when the Oni changes shape into an Ettin.
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Case 1: Oni's HP remain at 34
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Case 2: Oni's HP are now 85 as an Ettin
I am guessing the former
Other than its size, its statistics are the same in each form.
but am not sure. There does not seem to be a limit on the number of times an Oni can change shape, and it has another feature
Regeneration. The oni regains 10 hit points at the start of its turn if it has at least 1 hit point.
Possible Problem: Immortal Oni
If Case 2 is true, the Oni would seemingly never run out of HP as it changes shape at will when it gets low, and also regenerates.
Change Shape. The oni magically polymorphs into a Small or Medium humanoid, into a Large giant, or back into its true form. Other than its size, its statistics are the same in each form. The only equipment that is transformed is its glaive, which shrinks so that it can be wielded in humanoid form. If the oni dies, it reverts to its true form, (MM, Oni)
How does it work: Case 1, or Case 2?
Best Answer
Case 1, the Oni remains with 34HP.
As you quoted in Shapechange:
So all things in the Oni's stat block remain exactly the same when it transforms, apart from its size and appearance. This means he remains at 34HP, and regens 10HP per turn as usual.