The final boss, the Void Dragon, has the stats HOM_Dragon, which is probably found in the Shared version of Character.txt not the Main version.
If you are unsure, look up what stats characters use by loading the levels in the editor.
For some NPCs it might require a bit of looking, e.g. there are several versions of Leandra, and at least two versions of Icara, and there are many different versions of Arhu. But in normal cases, which are the vast majority, NPCs are usually placed in the area where you encounter them, the Void Dragon is in the level Homestead.
As I said, Vitality from individual entry and Vitality from the meta entry (HardcoreNPC) are added before they are used as multipliers. (Or the Vitality in the meta entry is a multiplier for the base hp again, amthematically, it does not matter.)
So when you change the meta entry to 2000, base HP should be multiplied by 21, even if you set the Vitality in the individual stat to 0, it is still a multiplier of 20 (from 100+2000 down to 0+2000), so of course you don't see very much difference.
Setting Vitality in the meta entry to a value of 2000% is crazy IMO, you have to make players totally overpowered to be able to survive the massive enemy HP. That makes fights take a lot longer but not necessarily harder.
So again, for most NPCs, HP is 100% of their base HP because of the Vitality 100 that most stats inherit from _Base. When an individual entry does not have a 'data "Vitality" "..."' entry, it inherits it from _Base. If such an entry exists, it will overwrite the value inherited from _Base.
(_Base is the entry at the very top of Character.txt)
In Tactician, the HP is 120% of the base HP, because 100+20 (from the meta entry) is 120 and that is the percentage multiplier. In Baardvark's Scales, HP for most NPCs is 200% if base HP, because Baardvark has Vitality 100, and 100+100 is 200 (200% of base HP).
100 in the meta entry is sufficient to double the HP of most NPCs, of ALL NPCs that do not have the Vitality of 100 inherited from _Base overwritten in their used stat.
If you want to double HP of ALL NPCs, you have to add 100 to the meta entry (HardcoreNPC) and adjust the Vitality in the individual stats to a value that is twice the value from before when 100 is added to them. For example if a stat has 'data "Vitality" "120", and you change the meta entry to 100, you have to double the part in the individual entry that is above the default 100, so changing the whole value to 140 will double HP for this individual entry too which had overwritten the 100 inherited from _Base. (140+100 = 240, which is twice the value of the 120 before.)
You can also set Vitality to 0 in the meta entry and instead set Vitality to 200 in the _Base entry and double all values that are overwritten in the individual stats, that is better control of individual stats.
The meta entries serve as a distinction between the difficulty modes, but due to how things work, it does not change everything as one would expect. The entry in meta is not a multiplier of current HP but is calculated from base HP again. This also means that NPCs using a stat with an already very high Vitality in their individual entry are affected A LOT less by it than those that do not have their Vitality overwritten or not much of an increase.
The Void Dragon already has 'data "Vitality" "500"', 500% of base HP, so adding 'data "Vitality" "20"' to HardcoreNPC brings this value to 520 which is of course almost the same.
If you want to double the Void Dragons health, you have to change hi entry to 900 if you change the meta entry to 100, or to 1000 if you change the meta entry to 0 or to 980 if you leave the default 20 for HardcoreNPC.
Changing individual entries though affects ALL difficulty modes.
With your change of the meta entry to 2000 and the additional knowledge that this is added to the multiplier, it is very clear that there is no way to bring the mob's HP down to normal values again, you have to use negative values in the individual entries. (Should work because it also works for "CasualNPC".)
You'd have to use a huge negative value to counter your huge positive one. (like -1900 to bring it down to +100 again)
But using negative values affects classic and explorer mode again, with a value as high as 2000 and the required adjustments in individual stats, all NPCs in classic and explorer probably end up with 1 HP, all a oneshot basically.
