I'm rather confused by the statblock for the Gnoll Vampire. The statblock states that:
Its statistics, other than its size and speed, are unchanged.
If this is the case, why is it specified later on that if the gnoll vampire dies in their hyena or mist form, they revert to their true form, wouldn't this not be the case if they had the same HP? What things change and do not change in Shapechanger form?
The gnoll vampire’s Shapechanger feature is not like a Druid’s Wild Shape or the polymorph spell. With those features, the new form has its own pool of hit points independent of the original form’s; for example, polymorph reads:
The target's game statistics, including mental ability scores, are replaced by the statistics of the chosen beast.
In contrast, the gnoll vampire’s hit points are unchanged when transforming:
Its statistics, other than its size and speed, are unchanged.
If it dies in hyena form, it reverts to vampire gnoll form (its “true form”), and is still dead.
The Gnoll vampire can shapechange from gnoll vampire form into that of a cloud of mist, or that of a hyena, but all that changes is the form of the body and the size and speed statistics.
In all these forms, it has the same statistics (the things printed in the stat block), including its hit points, other than size and speed. Those statistics include the actions it can take; for example, a Gnoll Vampire in mist form still has the action "Claws (Vampire form only)" as a statistic, but that statistic says it cannot use the claws in its current form. Nothing changes.
When it gets killed, for example because its hit points are reduced to zero, it will revert from mist or hyena form to vampire form - its hit points remain at 0, because they are a statistic, and do not change when it changes forms, so it remains dead.
