It says in the description that they are immune to magic and alchemical poisons. What is considered alchemical? I got bit by a spider that poisoned me, I know this is a natural poison but wouldnt alchemical poisons be made with natural ones?

Natural poisons are natural.

Alchemical potions are manmade.

You make alchemically potions by mixing chemicals, not by using a natural poison up.

Since NeverwinterNights doesn't distinguish between natural and alchemical poisons, our subrace system represents the Duergar immunity to magical and alchemical poisons by granting them immunity to the spell Poison.

ah gotcha