So I re-watched Twilight of the Apprentice parts 1 and 2 today, and I had a few thoughts regarding the Sith temple and the battlefield spreading out below and around it, particularly in reference to KOTOR II. 

Quick warning, I have done no research to see if there was ever an official statement which includes my theory, so I could be unintentionally restating established information. That said, on to the theory. I am also giving it the "Fan Fics" flair, because at the end it morphs into something akin to a brief fanfiction in my opinion. 

As is established in the episode, the Sith temple/battle station sits underground, surrounded by ruins and what appear to be legions of petrified warriors with their discarded and decaying lightsabers lying on the ground. This battle also likely was fought to the top, since not far from the central platform is the Jedi Temple Guard mask that Kanan wears in his duel with Maul. 

Now we come to the battle itself: pretty much all over the ground are frozen humanoid statues. The one closest to the camera (not sure if that's the proper word, since this is animation) is raising its arms in a way that would shield its eyes to keep from being blinded. The others are making similar motions, though closer to the ground. The two statues in the center of the image show one raising its left arm to its face while holding the right back as though preparing for a punch. Meanwhile the one in front of it is flipping into the air with one hand flat on the ground. 

In KOTOR II, the Sith temple on Malachor V does not look identical to the one in Rebels, but the basic positioning is the same: It is underground, with narrow archways and a glowing red platform at the center, and pillars curved to point toward the center while leaving an open space between them. As such, I think that if nothing else it is at least the inspiration for the temple pictured at the beginning of this post. 

Now for the crux of this theory: Darth Nihilus, aka the Lord of Hunger. In the KOTOR II storyline, he is established to have fought in the Battle of Malachor V during the Mandalorian Wars. To summarize the relevant details of the battle, the detonation of a superweapon called the Mass Shadow Generator destroyed all life on Malachor V and left Nihilus craving Force energy. He effectively fed on the Force and absorbed its energy from the people he killed, powerful enough to leave an entire planet drained of the Force, as he did to the planet of Katarr. For a demonstration of his abilities, I would recommend you watch the following fan film: 

With all of this said, I will now attempt to condense this information into a coherent explanation for my theory. 

I believe that the battle around the Sith temple on Malachor from Rebels is the same battle as in KOTOR II, through with a few minor differences. In the canon, the Jedi attacked in order to prevent the Sith from using the temple's destructive capabilities against them. They were winning the fight, going all the way to the top (probably using the Temple Guards as front line infantry, due to the mask being almost at the top of the temple). However, Darth Nihilus was waiting for the invaders, and he drained everyone on the planet of the Force. This would explain why the stone figures in the picture that I included act as they are trying to block some oncoming attack. In short, I think that the Battle of Malachor V is now, in a sense, canon. Maul then came thousands of years later in search of the holocron that was left behind (when he says that he will use the temple to have revenge on his enemies, he is probably trying to reawaken whatever mechanism or ritual the Jedi were fighting to prevent), and the Eighth Brother went to stop him, and possibly even to claim the knowledge Maul was after for himself. 

This leaves the voice that Ezra heard when he placed the holocron inside of the obelisk. In the Dark Empire comics, there is a Jedi holocron which Leia finds that contains the consciousness of the long-dead Jedi master Bodo Baas. Applying this information to the above theory, the voice may have belonged to a Sith that was present at or directly before the battle, creating the holocron as a store of information and key to weaponizing the temple, and when this proved no longer possible as the temple fell, tempting Ezra Bridger to use Dark Side powers in pursuit of his goals. 

Ok, I'm finished speculating. What are your thoughts? Do you think this is a realistic theory, does it make sense, or did I just spend a whole lot of time typing some that has already been established? Is there anything you would add/remove/change about it? I feel like this would make a great fanfic (hence why I gave it this flair) and would be very intrigued with the possibility of writing it.