Of the three Roivases, Edward's got the most magick, Max has the most health, and Alex has the most sanity. However, this is ironic, because the circumstances of their "defeat" are done in reverse the pattern of the Ancients, though Alex has it a bit differently.
Max has the most health, and thus the most power, but was driven mad. Chattur'gha represents power and normally beats Xel'lotath, because he can't be driven mad.
Bonus Points: he also has the lowest sanity out of the three Roivases.
Edward was the smartest of the three, and had the most knowledge, but he was eliminated by the power of another Ancient. Ulyaoth, who represents knowledge, beats Chattur'gha (power) in the normal pattern.
Bonus Points: Her sanity-meter gets lower with every chapter, the more she gains in knowledge, the more loose her sanity
Out of all the characters in the game, the ones with the lowest sanity are Peter Jacob and Michael Edwards. Why? Peter is struggling through World War One, while Michael Edwards is experiencing Gulf War Syndrome.
A certain Sanity-effect let's the head of a character explode, and if picked up it recites Hamlets famous "To be or not to be"-scene, in which he thinks about suicide. This enforces the theme of the game itself. The player can either go through the game the easy way and stop or go on and finish it with all the problems that have to come, including the sanity-effect.
Each Ancient, near the end of the game, reveals a vision they have of them beating the Ancient they're strong against. However, that never comes to pass, at least on that playthrough. They were seeing into an alternate timeline where Pious was aligned with the disadvantaged Ancient and Alex summoned the other Ancient to defeat it. All three of the Ancients saw these misleading visions apparently because of Mantorok's manipulations.
To advance in the tutorial level, Pious must at one point destroy a statue of himself - this serves to teach the player how to target specific body parts on enemies. When later characters go to the mausoleum to claim the Tome, his desecrated statue appears among all the others, which are intact. The others are all the heroes who have claimed the Tome and their place in its history, sacrificing themselves to ultimately destroy that timeline's Ancient. Pious' destruction of his own statue is symbolic of his fall from grace, as he then goes on to become the Liche, the Ancient's champion.
When when monsters look at you they drain your sanity, and when monsters are killed they fade away (as opposed to humans who slowly decompose). Both of these help the Ancient Conspiracy remain secret.
Peter Jacobs, Michael Edwards, and Edwin Lindsey are all Tomebearers, but their statues aren't seen in the Screaming Chapel. The only one of them who is even likely to be alive is Lindsey, who has a dangerous career anyway: why don't they have statues? They passed the Tome on to someone else before they died, unlike the others, who died either carrying the Tome themselves, or left it secreted away someplace where only they knew where to find it.
You'll also note they are the three Chosen who give Edward Roivas the three Ancients' essences. If Michael's last words in the game are anything to go by, people who handle these things in open defiance of an Ancient's guardians don't last long. Peter Jacobs would be the exception, but c'mon, he defeated the Black Guardian, they wouldn't want to go near him.
They are also the three most badass characters and the only ones who actually survive everything Pious sends against them (Max and Edward also survive their chapters, but are defeated by the Ancient's minions not long after), thus proving that they are suited to safeguard the Essences until they can be delivered to Edward.
It's possible Edwards is still alive, despite his comment suggesting otherwise, and in fact even probable - some random delivery boy wouldn't spook and book it from a mansion as quickly as the person delivering the third essence and possibly the Gladius did without prior knowledge of all the crazy things that happen there.
The first time Pious encounters a zombie, you can see that he's going for his sword before it rises. Between that, the fact he heard the ancients calling him and the lack of a sanity bar, it can be assumed that this wasn't his first time as a character in a Cosmic Horror Story.
Mantorok is the Corpse God; in his temple, there's a huge mural depicting some human tribe worshiping him, and the description describes him as a god of fertility and the earth. Well, naturally he would be: as a god with powers of death and decay, naturally fertilizer falls neatly under his realm of influence. It also serves as foreshadowing: Mantorok is easily the most benevolent of the Ancients. Its relationship with humanity is, and has always been, a symbiotic one.
As mentioned in the Flash Forward trope on the main page, Michael Edwards, the second to last of the Tome bearers, has knowledge of Edward Roivas's death, but does nothing to warn him. This makes perfect sense - Edward's death was instrumental in bringing Alex, the only person able to wield the enchanted Gladius and ultimately stop Pious, to investigate the mansion and learn the truth about the Eternal Darkness. Presumably, Michael acquired knowledge of his ultimate plan the moment he picked up the Tome, and was simply helping him carry it out.
In a Xel'lotath-aligned playthrough, she instructs Pious to "make sure [Charlemagne] is dead. Or insane... or perhaps one, then the other?" in her version of the cutscene before Chapter 3. Given what actually happens in Chapter 3, apparently Pious takes it directly to heart.
Detective Legrasse, who greets Alex at the beginning of the game? He is almost certainly a bonethief.
If he was, wouldn't he have stayed to try and kill her?
In Paul's chapter, he observes that the door to the crypt looks as if it's used often. One of the jewels needed to open it is found lodged in Anthony's eye socket... Is that where it's usually kept?
Edward Roivas can save his house staff from the vampire, which is awesome, but they still get attacked by a thing from beyond the veil. There's no way in Hell anyone wouldn't know the story of Crazy Max Roivas if they worked or lived in that house; for the ones that saw it? They know the story is true, which not only means they fully understand whatever ghostly presence they sense as Max traipses around the place as a spirit, but also that the corroboration that Max needed was there, just decades and decades out of reach.
When the enemies see you, they take your sanity, which is shown by their eyes glowing green. The Xel'lotath Guardian is a mangled pile of headless torsos, and when it takes your sanity, the holes where the heads should be glow green at you. It can see you with its neck stumps.