Duergar Constructs (MToF)
So, I need to elaborate a bit here. When we are talking about Duergar Constructs, we are not talking about Constructs built by the Duergar so much as we are talking about Duergar who have been turned into Constructs. In MToF, this was presented as some kind of Body-Horror punishment. In this book, they treat it more like an Iron Man suit. Which when worn grants fabulous powers and somehow changes their creature type from Humanoid (Dwarf) to Construct (Dwarf). This process is reversible (to an extent) with a short rest.
Duergar Hammerer
The Hammerer is a siege monster, allowing them to bash down walls and dig tunnels with relative ease. If you follow the DMGs object damage suggestions, it should take an average of two rounds for the Hammerer to smash a hole in a castle wall. Which leaves them digging a large sized tunnel in significantly less time.
The Hammerer has a melee reprisal attack, but despite being CR 2, is helpless against any kind of ranged attacks thanks to an abysmal 20’ movement speed. Which means its best use is digging a tunnel or breaching a wall to lead the attack. This will shift the encounter goal to being “Can you stop the hammerer before it breaches” instead of the traditional kill everything and then loot it.
In the changeover, the Hammerer had its Engine of Pain Reaction overhauled: No longer can you target the dwarf inside the machine, and it can follow up any melee attack with a single Hammer strike.
Duergar Screamer
Much like the Hammerer, the Screamer is a kind of digging machine that was reworked for war. Unlike the Hammerer, the Screamer isn’t a siege monster (honestly, that should have been a change) and instead attacks with a Drill and by using their AoE scream attack.
In combat, the Screamer is also helpless against ranged attacks due to being slow. However their Sonic Scream power can be used to knock melee enemies prone, which makes for a nice opening gambit after a wall has been breached. Or as a nice failsafe to prevent the PCs from reaching the Hammerer who is going to breach the wall.
The Screamer had the same changes to their Engine of Pain reaction that the Hammerer did, only they attack with a Drill instead of a Hammer.