So...you have a haunted house mission here.
While at first it seems well made, later on it becomes rather tedious.
It´s really hard to find all the loot. Maybe it´s just me but I didn´t find even enough for the loot goal.
Good thing this is optional.

What to expect? Here you are forced to sneak past enemies as all you will encounter are invincible.
While this adds a challenge that can be good, it´s getting frustrating if later on enemies can even one-shot you despite being out of their range.

But the major let-down here was the solution in the end. Not only is the object in question not something you can put in your inventory, no, it even defies physics.
Not really a mission I enjoyed.
BTW: When finishing the mission I was "rewarded" with a crash of the game.
2nd time it worked.