Beta Maxxthulhu is another Beta Maxx Cartridge, this time for a Weird Tales/Cosmic Horror style game broadly set from the Late 18th to Late 19th Century (which yeah is a heck of a wide window but anyway). This time your characters are a bunch of Adventurers who end up poking their nose into something maybe they really shouldn’t and finding something that will really rearrange how they think the universe really works.
This game changes from the original Beta Maxx systems in Beta Maxx X by adding in Fracture and Resolve. Fracture builds as you find out that nice Renaissance and Enlightenment understanding of the world could be utterly bogus; that the supernatural is very much real. This can then cause you to develop psychoses which in longer sort of campaign formats will result in ways your Characters can end up far out of control for a while.