Slaanesh is the Chaos God of pain and lust. He is the weakest of the four
In the pantheon of the major powers of Chaos, Slaanesh is the youngest and least powerful of the Chaos Gods, for the complex emotions that drive erotic desire, hedonism and decadence can only exist within the minds of intelligent mortal beings.
While generally referred to as a "he" by Humans and as a female by the Aeldari, Slaanesh is actually neither gender, combining characteristics of both and perfecting them.
Slaanesh's Daemonic cavalry option, Seekers' low armour but high armour piercing and poison damage makes them the epitome of this glass cannon army. Great at flanking, kiting, and isolating targets.
In reality, despite their grotesque appearance, the Beasts of Nurgle are extremely friendly and playful creatures, yet due to their natures as carriers of disease, they inevitably kill those that wish to be their friend.
Dark Eldar drink souls to stave off this leeching - perhaps by sating the thirst of Slaanesh, or perhaps by replenishing the essence of their own souls with that of the consumed one. Slaanesh will also consume the souls of Dark Eldar whole should they die.
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).
The forces of Slaanesh are led by Greater Daemons called the Keepers of Secrets, lithe creatures with unholy beauty. Slaanesh's realm is described to be a massive castle in the middle of seven circles, each representing one of the classical seven deadly sins.
Khorne is often referred to as a "he," with the masculine gender, though in fact, like all the entities composed of magical energy called the Chaos Gods, it in fact has no true gender.
Edward Lorenz was the father of "chaos theory," also known as the "butterfly effect." He explained how something as small as the flutter of a butterfly's wings could lead to a big change elsewhere in the world, like a tornado.
KHAOS (Chaos) was the first of the primordial gods (protogenoi) to emerge at the dawn of creation. She was followed in quick succession by Gaia (Gaea, Earth), Tartaros (the Pit Below) and Eros (Procreation). Khaos was the lower atmosphere which surrounds the earth--both the invisible air and the gloom of fog and mist.
Apophis, the Egyptian God of Chaos, was a snake – literally. His ultimate desire was to rid the world of order and return it to the chaos that reigned before the birth of the other gods. In most Egyptian fables, he is portrayed as the ultimate enemy of the sun god – Ra. What is this?
Tzeentch is second most evil and dangerous God because of his jokerish cruelty because he treats his followers as nothing more than pawns but far more benevolent than The Prince/sses.
That name is Malice, the Renegade God of Chaos. In eons past Malice was cast out from the bosom of Chaos by the other Gods, or else abandoned them of his own volition, no one is sure which.
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.
Slaanesh is the most sadistic and depraved of the Chaos Gods and he consumes the souls of all Eldar after death to torture them for eternity (for that reason, all Eldar carry soulstones that capture their souls). Slaanesh also encourages things like extremely depraved forms of rape, torture, slavery, etc.
I know Tzeentch and Nurgle are rivals, but why exactly are they considered so? Would their followers (chaos marines following their respective deity for example) actively fight one another if they encountered one another?
The oldest and first of the chaos god is Khorne, this is said repeatedly in the Liber Chaotica which is our best in universe canon understanding of how the forces of chaos function.
According to Hesiod, Chaos independently produced two children: Nyx (“Night”) and Erebus (“Darkness”).
According to Hyginus: "From Mist (Caligo) came Chaos. From Chaos and Mist, came Night (Nox), Day (Dies), Darkness (Erebus), and Ether (Aether)." An Orphic tradition apparently had Chaos as the son of Chronus and Ananke.
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.
They fear a lack of stimulus. One of the Salamanders novels, one of the characters threatened a Dark Eldar by locking it in a dark cell with no light and giving it barley enough stimulus to survive, but nothing more.
The Champions of Slaanesh are the Chaos Champions and Chaos Lords of Slaanesh, the Chaos God of pleasure and pain. They are every bit as decadent as their Chaos Cultist kin in the south, nimble and fast duellists that slither through enemy blows like a striking viper.
Falling to Chaos would be a death sentence to an Eldar, as they would eventually be claimed by Slaanesh and their souls consumed to sate her unending hunger.