From the introduction:
Against Tsathogga is a short, high level adventure designed to allow the GM to use the massive Tsathogga miniature from Center Stage Miniatures. That’s it. We had no real meta-world need for such a ridiculous adventure module, but how often do you really have a high-level party needing an adventure module that gives you an excuse to hit the table with two pounds of resin frog?
This adventure is “suitable” for 4–6 PCs of level 16+. Don’t kid yourself; the PCs have NO chance against Tsathogga if he is summoned. If your party is larger or of higher level, adjust the numbers of tsathar priests and scourges available in the final encounter. Don’t add levels if you don’t feel the need; it’s a lot of work for nothing. The tsathar don’t need to win the combat, they just need to delay the PCs long enough for the Ranan to finish his summoning. The PCs shouldn’t be able to stop the summoning, but if they do, that is up to you as a GM how to proceed. You got this module to use Tsathogga, so you might as well fudge to let the PCs see it! They know it’s coming; give it to them.