This mission has a high rating for a reason. Creative use of brushwork and a more complete feel than most FMs of it's time both make this mission significantly stronger than you might expect.

You are breaking into an abandonned hammerite cathedral, in order to decend into it's depths, and take out a pagan cult. Each difficuilty comes with it's own objectives, and on the expert equivalent, you are expected to kill the cult leader, and generally to be a bit murdery. This is the difficuilty I played on.

Atmospherically, the early parts of this mission are spectacular. The level design is a bit ropey outside, but once you find your way into the cathedral, the whole place just feels right. While this isn't fully maintained through the whole mission, it's maintained enough that this is definitely it's most important aspect.
Then we have have gameplay. Slightly weaker, but still very much top of it's class for when it was released. A few nasty suprises here and there, and just enough puzzle elements to keep it interesting.

Story wise, it ties directly into T1s primary plot. It is executed fairly well, but not as strongly as the other 2 aspects. Enviromental storytelling is not particularly common, but it's functional, and there was never anything that made me sctratch my head, or disbelieve the world.

All in all, just a very solid FM.