The players come into the area for an unrelated mission, and during the adventure another group of npcs gets captured by the minions and brought to the lake side, where they are tied/impaled to stakes ready for conversion to undead minions by the god in the lake. Once this piece is delivered to the lake the god will have enough power to be free of the lake. It's underground and they're digging it up. It has located the final and largest part of the meteorite. It's had a particular group of minions for a 100 or 200 years, and uses them to find parts of the meteorite (which broke up and scattered). It can influence people who come close to the lakes, and persuade them to come to the lake and then convert them into undead minions (intelligent, not mindless). The big bad is an alien/god that crashed to earth in a metorite prison millenia ago, and has been trapped in a lake/group of lakes in the mountains.

It sounds great, but I want to introduce my players to wfrp, not CoC, so I'm converting it to wfrp 2e. The adventure named in the title ('AAT') is in the CoC 7e core rulebook.