Multiattack. The screamer makes one Drill attack, and it uses Sonic Scream.
Drill. Melee Weapon Attack: +6 to hit, reach 5 ft., one target. Hit:10 (1d12 + 4) piercing damage.
Sonic Scream. The screamer emits destructive energy in a 15-foot cube. Each creature in that area must succeed on a DC 11 Strength saving throw or take 7 (2d6) thunder damage and be knocked prone.
Engine of Pain. Immediately after a creature within 5 feet of the screamer hits it with an attack roll, the screamer makes a Drill attack against that creature.
Creative duergar engineers have built numerous war machines, including some that can be fused with a duergar. Such a duergar-machine hybrid is fueled by the duergar's psionic energy, and the duergar inside the machine can psychically channel pain into power when attacked.