I'm planning a campaign with an underdark-based homebrew setting, and I want to give them the option of Duergar as a race. Does this seem unbalanced, and are there any potential issues in particular that I should watch out for?
As a subrace of dwarves, Duergar have all the standard Dwarven racial traits. In addition, you have the following traits:
Ability Score Increase. Your Strength score increases by 1.
Superior Darkvision. (as per Drow)
Sunlight Sensitivity. (as per Drow)
Duergar Resilience. You have advantage on saving throws to resist being charmed or paralyzed. You have advantage on Wisdom, Charisma, and Intelligence saving throws against spells. You have advantage on Intelligence (Investigation) checks made to discern illusions.
Duergar Magic. When you reach 3rd level, you can cast the enlarge/reduce spell once, but only to enlarge yourself. You regain the ability to do so when you finish a long rest. When you reach 5th level, you can cast the invisibility spell once, but only on yourself. You regain the ability to do so when you finish a long rest. Intelligence is your spellcasting ability for these spells.