I think Duergars are a bit weakened compared to their cousins the Dwarves.
Let's see the stats:
Dwarf:
21 STR 18 INT 21 WIS 18 DEX 25 CON
Duergar:
21 STR 17 INT 20 WIS 20 DEX 22 CON
That is, Duergars lose 1 INT/WIS and 3 CON for a mere +2 DEX.
But for those of you who think this is a minor effect, i will tell you the difference in the final HP/Mana.
A Duergar cleric will get around less 180 HP and 150 Mana base than a dwarf.
I know being evil is very cool, but does it justifies this kind of penalty based on alignment only ?
Can we please at least improve the mentals ?
Being as smart as a giant and less smart than a Ogre is quite strange for a Duergar.
Perhaps a quick +2 INT would be a quick fix. That way they are not as wise as a Dwarf, but are not mentally challenged. Call it evil cunning. And perhaps a +1 Con.
