MMM If i remember, HD in D&D went by 8. A 2 HD monster would have 2d8 hp, or 2-16 total ( 1d8 + 1d8 )

Now that is only by monster level. a 20 HD monster would be 20d8, or max of 160 hp. Im sure in BG2 most are maxed. Damaging spells, HP for characters, etc can be different amounts. Monsters are all the same, 1d8 per level, but player characters and ingame characters who aren't monsters (drow, Npc's) use their regular HD. Thus a 10th level Drow fighter would have 10d10 hp, a Drow cleric would have 10d8, you get the idea.

Ok. It differs in 3rd Edition, but BG2 is mostly 2nd Edition AD&D:
.
Monsters use D8 for hitdice
Wizards/Sorcerors use D4
Thieves/Bards use D6
Clerics/Druids use D8
Fighter/Rangers/Paladins use D10
Barbarians use D12

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