Though even dark vision (for all but Drow) isn't always enough if you're fighting at range. Most darkvision only has a range of 12m from memory whereas maximum range of many spells is 18, and bows even longer if youre up high. Not sure what range of darkvision most of your opponents in the Underdark have?
If they add Duergar and Svirneblin they will have 24 m dark vision.
