Armor Class 16 (armor of the insane)
Saving Throws WIS +4
Skills Arcana +9, Perception +3 (due to mad insight), Stealth +5
Magic Resistance. The derro has advantage on saving throws against spells and other magical effects.
Sunlight Sensitivity. While in sunlight, the derro has disadvantage on attack rolls, as well as on Wisdom (Perception) checks that rely on sight.
Actions
Bolt of Chaos. Ranged Spell Attack: +7 to hit, range 120 ft., one target. Hit: 14 (4d6) damage of a random type (roll d4: 1=necrotic, 2=poison, 3=cold, 4=lightning) and the target must make a DC 15 Constitution saving throw. If they fail, the target suffers a special effect that depends on the bolt's damage type:
Mind Scourge. The chaos savant targets one creature within 60 feet, who takes 10 (3d6) psychic damage and must make a DC 15 Intelligence saving throw. If the save fails, then at the start of the target's next turn it must choose between taking an additional 10 (3d6) psychic damage or being stunned until the end of that turn.
Window to Madness (Recharge 5-6). The chaos savant targets one creature within 60 feet. The targeted creature must succeed at a DC 15 Wisdom or Charisma saving throw (target's choice) or take 10 (3d6) psychic damage and become crazed for 1 minute. All other creatures within 10 feet of the creature targeted by Window to Madness must succeed at a DC 15 Wisdom or Charisma saving throw (creature's choice) or take 5 (1d10) psychic damage and be crazed until the end of their next turn. Derro are immune to psychic damage caused by Window to Madness but can become crazed.
A crazed creature has disadvantage on any ability check to interact socially with creatures who are not crazed or insane (such as derro).
The crazed target must make a DC 15 Wisdom or Charisma saving throw (target's choice) at the start of each turn, ending the effect on itself on a success. If the save fails, the target must choose whether to take 5 (1d10) psychic damage or become Dancing Mad (see below) until the start of its next turn. If the target fails the saving throw by 5 or more, it takes 5 (1d10) psychic damage and is Dancing Mad that turn.
Bonus Actions
Dance of Madness. The chaos savant targets a creature within 130 feet that is crazed by one of its Madness attacks. Each creature in a 10-foot-radius sphere centered on that target must succeed at a DC 15 Wisdom or Charisma saving throw (creature's choice) or take 5 (1d10) psychic damage and become crazed and dancing mad (see above) until the start of its next turn. Derro are immune to psychic damage caused by Dance of Madness.
Strength of Madness (Recharge 4-6). Until the end of the chaos savant's turn, the savant gains advantage on melee attack rolls and its melee weapons do an additional 4 (1d8) damage if they hit.