In the millennia since his ascension to the Golden Throne, the Emperor has become a god to his people, the worship of Him uniting humanity throughout his Imperium, superstition and dogma replacing his doctrine of Imperial Truth.
The emperor never wanted to be seen as a divine being, he didn't want to be worshipped. His imperial truth was to cast down all religion throughout his growing empire, to replace the need for worship ( which inevetingly fed the ruinous powers because of the emotions involved in doing so).
He is a major warp presence that's so powerful he can grant gifts to his followers (see Sisters of Battle as a great example). The main difference is that he is ultimate Order where the Chaos Gods are ultimate… uh… well… chaos. However, given his presence in the Warp, he is often described as Mankind's chaos god.
The four main Gods of Chaos are Khorne, the blood god who delights in primal violence, Tzeentch, the lord of change who relies on sorceries and knowledge to further ambitions, Nurgle, the plague father who represents disease, death, and rebirth, and finally Slaanesh, the dark prince of pleasure.
Theoretical, Guilliman thought savagely now, turning his anger against his traitorous mind. The Emperor was never a god, denied He was a god and has been wrongly elevated by men who see power and mistake it for divinity. Practical, the Emperor is not a god. 'He is not a god,' said Guilliman out loud.
Lorgar absolutely believed the Emperor was a god with every fiber of his being, even after his stunning rebuke on Monarchia. When he learned the truth about Chaos, this belief still hadn't left him though he now knew there were other great powers in the universe.
After nearly being imprisoned for eternity in the Maelstrom and then almost slain by his brother the Daemon Primarch Magnus the Red on Luna, Guilliman finally arrived at Holy Terra.
The Orks can also be considered to be the most successful and powerful in Warhammer 40K due to their partial immunity to chaos, ability to reproduce quickly, and ever-growing strength when facing powerful threats.
Khorne hates magic and those that use it, thus reserves much of his hatred for tzeench but his true wrath is reserved for Slaanesh - who is his complete opposite and seen as a weakling by the war-god. He is associated with the number eight, and it's reflected in rituals.
Since the Emperor was already god-like, 10 thousand years of worship has scaled him up some. This is actually what the chaos gods were afraid of. They couldn't resist him now even if they united. Fortunately for them, the Emperor foolishly imprisoned himself in the Golden Throne.
Slaanesh is the Chaos God of pain and lust. He is the weakest of the four chaos gods and the fastest fighter from his speedster movement. He was created from the Eldar races past carefree nature after getting bored conquering their enemies they deemed themselves superior over.
In the book Godblight, Guilliman's Primarch brother Mortarion pulled him into the Warp to slay him in Nurgle's garden. But the Emperor was able to heal Guilliman, use him as a vessel to burn down Nurgle's gardens, and then revive his son fully — which is a power play that we're not used to seeing from the big guy.
The name Seth (or Satet) means strength, but in ancient Egypt, this god also stood for chaos, destruction, confusion, evil, anger, and storms.
It seems to me that the novels are leading up to the fact that the Emperor held back not because he loved his son and didn't want to destroy him, but because he had stopped caring. His multiple millennium plan had been destroyed.
However, the Emperor was immortal, and continued to live, in secret and in shadow, for millennia. This is where things get interesting: the Emperor entered the Gateway into the realm of Chaos and there met with the Dark Gods, who bestowed their power upon him in exchange for the Emperor spreading Chaos into humanity.
During this ancient time, the Emperor was known as Neoth. At some point before recorded history, the Emperor became a king and organized his own armies with the Perpetual that will become known as Ollanius Persson as his Warmaster.
Khorne respects Nurgle's power, and as Khorne respects power unto itself, Khorne allies with Nurgle more often than not. Nurgle, described as an almost jovial god, does not hate any of his brother gods, it is to him that they all will eventually waste away.
Skarbrand, also known as the "Exiled One," the "Wrathful Reaper," or the "Drinker of Blood," is an Exalted Bloodthirster of Khorne, once the mightiest of the Blood God's Greater Daemons. His arrogance, hubris and sheer rage, secretly fanned by Tzeentch, grew so great that he actually attacked his Chaos God patron.
He hates all other deities, including the other Chaos Gods (particularly Slaanesh as it is Khorne's opposite). From what I've seen in W40k related games I've played and backstory from my 4th edition Rulebook (I don't own many 40k books ATM), he seems to hate the God-Emperor of Mankind more than the other Chaos Gods.
The Daemonhost is arguably the strongest enemy in Warhammer 40K: Darktide. Her attacks are incredibly fast and deal a lot of damage.
Lord Nemeroth (also known simply as Nemeroth) is the main antagonist of the Warhammer 40k: Space Marine video game. He is a powerful chaos sorcerer lord and leader of the Chaos Space Marine warband the "Chosen of Nemeroth", invading the Forge World Graia through a warp gate.
When he was restored to life by the combined skills and knowledge of Belisarius Cawl and Yvraine, Roboute Guilliman was told that if he ever removed the life-giving Armour of Fate, he would die.
' The Emperor loves no one man, thought Guilliman. He cannot afford affection - that is the honest practical for the impossible task that faces the Master of Mankind. He did not love His sons, He does not love men, but He does love mankind.
So According to the new Dark Imperium Novel "Godblight", the Emperor himself set the entire realm of Nurgle on fire. The Plague God screamed in horror and rage. But Grey Knight Supreme Grand Master Kaldor Draigo has also done this in the past and the garden regrew very quickly.