About Rashalga…
The foundation is an odd place - the corpses do indeed look like aether tainted bodies, but it may just be that the mountains of bodies are, from a code and design perspective, recycled assets from other areas of the game. If memory serves correctly, Lucius’ boss room and Anasteria’s boss room (if you fight her) are filled with similar piles of fleshwarped bodies. Also what strikes me as odd about the Foundation is the pond of fresh blood inside Rashalga’s area - as an odd contrast to how much of the game’s environments are liberally strewn with guts and viscera, the Foundation of Corpses is one of the only places that has actual pools/puddles of blood, with the only other place I know of being one certain area inside the Temple of the Three.
I’m under the impression that the Foundation of Corpses has some sort of naturally-formed cave formation connecting its ceiling to the surface, and that Aetherwarped creatures who weren’t familiar with the Asterkarn’s terrain have been accidentally falling into the cave over the course of the Grim Dawn: my thinking is that they’re left mortally injured by the fall, after which point the Mad Queen kills them or just lets them drown in the blood pond. At the very least, I’m not convinced that Rashalga killed all those creatures to feed on them, considering how careless she is in letting them rot/lose all their nutritious fluids.
At the very least, that’s the most likely explanation I can think of for how her chamber is filled with freshly-dead bodies if she’s apparently trapped in there.
Of course, I’m personally more keen on believing that the Foundation of Corpses’ design is more for the players’ sake: Considering how lethal Rashalga is, it’s appropriate that her area needs to appear overtly, exceedingly dangerous.
About the Temple of the Three
The Sentinel does seem to be a mystery. The Attendant doesn’t seem to acknowledge the Sentinel at all in dialogue, but everything about The Sentinel seems to suggest that he/she/they/it served as some sort of guard appointed by the Witch Gods to defend the temple. I don’t think the Sentinel is or was ever a human at any point, since The Attendant’s dialogue gives me the impression that the Eldritch Gods don’t reward their human followers with that sort of power.
One question I have to ask is how the Occultist abilities fit into the lore context of the Witch Gods going ‘missing’ - for instance, the description of the “Possession” skill states that the Witch God Solael essentially assumes direct control of your character, which seems a bit odd if he’s apparently ‘missing’ according to anecdotes from his previous devotees. Similarly, the fact that the ‘Eye of Dreeg’ skill works perfectly fine implies that Dreeg is still capable of influencing Cairn in some shape or form, even if his covens were unable to commune with him directly.