Reviving my BZPower account only to have this thread be the first thing I read after probably 10+ years of inactivity... Love it.
The premise can be divided into three conditions:
1. Toa Tahu (Mata) jumps into a volcano
2. A really dangerous secret cult is within the volcano
3. Toa Tahu (Mata) manages to single-handedly defeat the really dangerous secret cult.
Given that the title specifically asks how crazy it would be for Toa Tahu (Toa Mata) to land himself in this scenario, we have a wide but defined timespan for when the premise would at all be possible: it must be after Tahu arrives on the shores of Mata Nui, and before the decisive battle against the Bahrag.
Within the boundaries of this timeline, Tahu undertakes a quest to retrieve his six Great Kanohi. Being able to surf on lava, and having a well-documented habit of daring feats, Tahu may well have investigated the innards of a volcano in pursuit of his aforementioned objective. Toa Tahu (Mata) jumping into a volcano would not, in itself, be crazy.
As for the existence of a really dangerous secret cult frequenting/living inside a volcano, to my knowledge there is no confirmed canon that supports the existence of this phenomenon. What we do have, however, is Ahkmou. Ahkmou is a recorded instance of a Matoran* being secretly in league with forces hostile to the ways of the Great Spirit. This does open up for the possibility that there may exist other Matoran who are secretly in allegiance with forces whose goals are incompatible with that of the Toa. Mata Nui is, ideologically, a monoculture--there seems to be no officially declared differences in philosophical or ethical matters between cultures, and so it is to be expected that any subculture that does uphold fundamentally different ideals than the norm would be acting in secrecy, rather than out in the open. As for the choice of a volcano, lava surfing is an activity available not only to Tahu, but to Ta-Matoran** overall. For now, we can conclude that the idea that there would exist a secret cult of Ta-Matoran in secret league with some subversive force is unexpected and unfounded, but not impossible--I would label this as wild but not crazy.
The cult in this scenario is supposed to be really dangerous, but still get beaten by Tahu and Tahu alone. This is the most difficult condition to test, since 'really dangerous' is such a nebulous concept. Ahkmou for example was really dangerous in the sense that if left undiscovered, he could have kept up the plague in Po-Koro (and possibly spread it further), but if discovered he would not stand a chance against a single Toa, ie Tahu. Does their status as 'really dangerous' get revoked the moment Tahu appears, since their undisclosed machinations are no longer protected by the element of secrecy? Of course, one might also imagine that even a Toa might, if alone, face a decent challenge against a numerous enough force of antagonistic Matoran. Perhaps they are gifted with the knowledge of how to create the mindwarping sludge that infects masks, and are using the volcano to produce this substance? If so, Tahu could reasonably find the encounter 'really dangerous' despite being powerful enough to end them on his own.
Conclusion: If this would have happened in some canon material, with a setup like the one suggested above or similar, I would not have questioned it for a second. Would have been wild, but not crazy. On the other hand, if someone like Greg F was to post an update to his social media today--decades after the Mata era--and say "oh btw, it is canon that Toa Tahu during his Mata years jumped into a volcano and found a really dangerous secret cult that he also defeated singlehandedly, we just never showed that", I would think "****** that's crazy".
*While the premise did not require the cult to be made up of Matoran, I cannot think of any other lifeform established on Mata Nui at the time that would have the sapience and cultural context to form such a thing as a 'cult'--hence, my decision to work from the assumption that the cult consists of Matoran.
**Whether a non-Ta-Matoran can lava surf, I'll leave for someone else to fill in.
