I also fail to see why the human hydra is there, I guess its for the laughs
Might be the same as the Altar of darkness - sacrificing party members to it gives nothing in return and praying to the God Of Depth gives no benefit either. Malevolent creatures don't really like to share, it seems. That makes me think - perhaps the Old Gods chose to be unresponsive to humans BECAUSE they stopped making actual requests to them and the new gods, while being presented as the heralds of salvation, provide no actual help at all as they have no wish to squander their newly acquired godly powers.
Also, thanks for the feedback, i'll make a mercenary and try to study the Ma'Habre's layout - I'll try to go deeper ignoring all the chests and barrels and see what it has to offer.