Cegorach
Cegorach is the Eldar Laughing God and is a master of Just as planned and is also a massive dick. His dickery is only rivaled by The Deceiver and Tzeentch. He's the Patron god of the Eldar Harlequins, and probably Eldrad who looked up to Cegorach's dickishness and general jackassery as a shining example for all Eldar Farseers. He plays games of Just as planned with Tzeentch, the Deceiver and the Emprah every saturday night; it always ends in a tie. He's also one of the only surviving Eldar Gods, the other being Isha (Who's with Nurgle) and Khaine, but he's currently in a Bajillion pieces so in spirit, he's not "alive" anymore.
History
During the war against the Necrons, Cegorach convinced the Outsider to eat other C'tan, which caused the Outsider to become insane because all the C'tan he ate became split personalities within his body. But there is a huge argument as to whether he convinced the Outsider to eat the C'tan first, or if he was just copying the Deceiver when he told the Nightbringer to eat other C'tan.
After the C'tan and the Necrons went to sleep, the Eldar pretty much ruled the entire galaxy and because they were so advanced that they didn't have to do any work they started to do whatever the hell they wanted; kind of like The Culture but more Grimdark. At first it was just video games, amusement parks, walks in the park with SPESS ELF Dogs, sports, movies, fighting the occassional Aliens who wanted to challenge them (which ended in the extinction of most of these aliens save for the Orks and later humanity),somewhat increased amounts of recreational sex, and television but because the Eldar experience emotions way more strongly than people and time goes by slower for them, and at that point any time an Eldar died they would simply reincarnate in a new body they got bored of wholesome fun very quickly.
They started to do drugs, then they started having orgies, then they started with the BDSM, and it only got worse and worse and even some of the Eldar Gods started to get in on the "fun." Cegorach was far too busy hosting the eldar version of Monty Python and being a dick to join in on the hedonism and used it as joke fodder. When Slaanesh was born and went on his/her/it's/hermaphrodite's killing and raping spree (powered by Chaos performance enhancing drugs) and killed nearly all of the Eldar Gods, created the Eye of terror which swallowed the core worlds of the Eldar empire and sent a psychic scream that killed most of the Eldar race; Cegorach hid behind the stronger gods and gave Slaanesh the occassional sucker punch and the (not so) occasional devastating one-liner.
Soon after killing Asuryan; the strongest of the Eldar gods, Slaanesh's performance enhancing drugs started to wear off and while he/she/it/hermaphrodite wasted time violently raping Isha, Khaela Mensha Khaine got his shit in motion and started to fight Slaanesh. Slaanesh had a hard time of it but soon Khorne came out of nowhere to fight the prissy little faggot because he rightfully claimed that Slaanesh had no right to lay a hand on his property, which in this case was Khaine. Cegorach took this time to sneak into the webway while no one was looking and the rest is history.
The Harlequins
Cegorach is the patron of the Harlequins and keeper of the Black Library which Ahriman really wants to get into. Sometimes an Eldar with no soul is [s]born[/s] made during the Ritual (which is, obviously, an entering trial - being possessed with a Greater Daemon of Slaanesh and in ability to expel it automatically confirms that you are an incredible badass), which the Harlequins refer to as a Solitaire, these Eldar have great power but they are doomed to be eaten by Slaanesh once they die. While they live, they can assrape any army you throw at them. Seriously. But Slaanesh never gets to nom one because each time he/she/it/hermaphrodite tries, Cegorach punches him/her/it/hermaphrodite in the face repeatedly and gives him/her/it/hermaphrodite a devastating insult that causes Slaanesh to run back to his/her/it's/hermaphrodite's palace in tears. Happy end.