Construct Body: "Miru"

To get Miru go to The Etheric Vault in Maldraxxus, the entrance is at 24.5 31.63

Once inside the Etheric Vault go right to the second alcove and pull the 'Cell Portcullis Chain' at 58.63 35.66

After the chain is pulled you will receive a message 'Somewhere a portcullis has been raised.' Going back to the central entrance chamber and going down the central passage in a north-west direction to the chamber on the left at the end of the hall at 33.43 32.37, this is where you will find [npc=166096] and the soul will have a quest to complete.

Accept the quest A Lost Soul, go back to the chamber in the north-east of the Etheric Vault (67.5 15.75) and kill [npc=162371] to complete the quest, then turn in to [npc=166096].

Head back to your Stitchyard and find [npc=167877] at 54.95 68.07, accept it's quest Soul Survivor, interact with [npc=167877] and turn in the quest to [npc=167150]. Then use the [npc=167042] to build Miru's construct.

tldr:
Head to the Etheric Vault:

/way Maldraxxus 24.5 31.63 Miru (Entrance to Etheric Vault)

Once inside the Etheric Vault use the following waypoints:

/way 58.63 35.66 Step 1: Miru (Cell Portcullis Chain)
/way 33.43 32.37 Step 2: Miru Soulblossom
/way 67.5 15.75 Step 3: Bacilia The Lost

/way Maldraxxus 54.95 68.07 Miru Soulblossom (Stitchyard)

It's not bugged. You can only pull one chain a day and there are two chains. One to the left and then north when you enter and one in one of the two cells to the right. If you've pulled the correct one then the gate will be open

You gain 15% health when "Soul Swapped" with Miru and makes you look like him and him look like you.

I looked at "all" the character stats using DejaChartacterStats and didn't see any other numbers that changed.

I tried attacking a training dummy and as far as I could tell Miru does no damage (swapped or not).

As with all constructs (at the time of this writing) he cannot be used in The Maw, Torghast or pre-Shadowlands content.