This is just a short adventure, set before the current story arc. It works as a stand alone story. It's only a 16 page book, Monstrum issues usually run at 52, although issue 3 looks set to be even longer.
The story is based on Issue 2 Vol 1 of Monstrum, which I think was released in 2004. It only had a tiny print run, so I thought it was worth reviving the storyline and updating the artwork and giving it a bigger release.
Click here for more Monstrum Horrendum adventures.