Despite His early dealings with them, the Chaos Gods themselves later recognised the Emperor as their greatest enemy among all the intelligent beings of the galaxy, naming him "the Anathema."
The Emperor for a long time was (maybe still is) the most powerful being in the galaxy. The Gods of Chaos FEAR him, and his mission. His mission being to unite the galaxy under one flag, open the web way project, and close the warp.
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.
No. While he is a psychic entity that draws power from worship and sacrifice, much like the Chaos gods, he is more akin to a god of Law, than anything like a 5th Chaos power.
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.
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.
Stephen Sutton's answer is exactly right - Khorne will never forgive Skarbrand. More importantly, and more specifically, Khorne can never forgive Skarbrand. To say that he is “incapable” of forgiveness or mercy doesnt really do justice to the situation. Khorne is a being composed of pure hate and rage.
Despite His early dealings with them, the Chaos Gods themselves later recognised the Emperor as their greatest enemy among all the intelligent beings of the galaxy, naming him "the Anathema."
5 Malice/Malal
Oh yes, there's a fifth chaos god. Though not considered one of the great four, Malice stands apart from the others due to being known as a bit of a renegade. Also known as the lost god, Malice represents anarchy, destruction, and vengeance (even against itself and its agents that serve the cause).
The One Above All can and should be able to defeat The Chaos King with ease. The One Above All IS the most powerful in the Marvel Universe. He has not been defeated and literally will never be defeated for the days to come. His power can't even be imagined by anyone, it's just too much to comprehend.
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.
Khorne is generally the most powerful, though Nurgle also becomes strong regularly. According to the 3.5 codex, though Slaanesh could challenge Khorne briefly, the very nature of its power would cause it to burn out long before Khorne.
All the Chaos gods are rivals, and they all dislike each other in the competition for dominance in their great game.
Powers and abilities
Thanos wielding the Infinity Gauntlet ranked Lord Chaos' scale of power as above that of Galactus, but below that of Eternity. However later he and Master Order were able to easily kill Living Tribunal by blasting him with energy and also defeat Lifebringer Galactus after transforming into Logos.
Chaos was a male. He was a chaotic mass of elements. Chaos was said to unite with his daughter, Nox (Nyx in Greek), and they created the Island of Creation. They created Terra (Gaia) and Caelus (Ouranos).
In the book Vengeful Spirit, page 331, it says the emperor got his powers because he traveled to the planet Molech and entered a warp gate to speak with the Chaos Gods. It then says that the emperor made a deal with the Chaos gods, who gave him psychic powers that his has today.
So while Chaos is infinitely more powerful than Zeus, that is not to diminish the power of the “King of the Gods,” who could be called the most powerful of the corporeal beings in the universe.
Slaanesh is the youngest of the Chaos Gods, birthed by the fall of the ancient empire of the Aeldari.
The Emperor's Children are the primary Slaanesh-aligned faction of Chaos Space Marines, and potentially the strongest of the legions. Masters of martial perfection, they have a ton of great rules and Stratagems, plus they're the only legion which can take Noise Marines as Troops.
As far as I have understood, Khorne and the Emperor are by far the strongest beings in the WH40K universe, making the other Chaos Gods and other powerful beings look like insects in comparison; they could destroy anything or anyone in the blink of an eye, except for each other, because they both grow in power all the ...
In Vengeful Spirit, the Emperor and Alivia Sureka travel to Molech and enter a Gateway to the Realm of Chaos. Here, the Emperor makes a pact with the Chaos Gods where he gets power in return for spreading Chaos worship throughout Humanity, but immediately reneges on his end of the bargain and becomes the Anathema.
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.
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.
An'ggrath the Unbound, the mightiest of the Bloodthirsters, is the most favoured servant of the Blood God Khorne. In the pantheon of the Blood God's servants, An'ggrath stands next to the Skull Throne at Khorne's right hand.