NOTE: Some sources may suggest that Malal does in fact exist, although the facts are not certain, and any & all evidence is under dispute. This article presupposes that Malal exists, and all statements should be considered theoretical. And extra heretical!
Followers of Malal are kind of like Chaos Agnostics. They doubt that anything exists, including the Emperor, God, Chaos itself, and you. Especially you. The Chaos Space Marines chapter known as the "Sons of Malice", with their alternating black/white colour scheme are as likely to kill Chaos Forces as anyone else on the field, except for each other.
- "...and he that went before now came last, and that which was white and black and all direction was thrown against itself. Grown mightily indignant at the words of the Gods, Malal did turn his heart against them and flee into the chambers of space . . . And no man looked to Malal then, save those that serve that which they hate, who smile upon their misfortune, and who bear no love save for the damned. At such times as a warrior's heart turns to Malal, all Gods of Chaos grow fearful, and the laughter of the Outcast God fills the tomb of space . . ."
- -- from The Great Book of Despair.
So anyway, Malal, when he was actually canon, was the Chaos God of batshit loony self-destructive urges; Chaos battling Chaos. This also made him the God of paradoxes, radical Inquisitors and the like trying to turn Chaos against itself, and the outcast god since he was trying to buttfuck every other Chaos God and their followers. The thing about Malal was that even though he was one of the biggest personifications of Chaos there could be he constantly tried to destroy Chaos and if he were ever successful in ending Chaos he would be destroyed as well. Not that this pants-on head crazy a-hole cared, as suicidal tendencies and teamkilling just became part of his portfolio instead.
Since Malal was supposed to be the antitheses of Chaos he had only a few champions, all of whom were supposed to be stupid powerful and would go around bitch-slapping other Chaos champions with their anti-daemon daemon-axes of doom while wearing warp-resistant warp armor. The servants of Malal fight in utter silence, but that makes them way badass than the rest of chaos. So yeah. Because of this followers of Malal have to already be balls to the wall nuts and have superhuman will. Malal's trademarks were black and white bisecting armor and a horned skull equally bisected black and white. His sacred number was 11. His signature weapon was the dreadaxe which was a daemon weapon made out of a daemon that hates daemons, and it looked like a pterodactyl head on a stick. You can still find examples of this weapon in the CSM 3.5 dex and in Your Spiritual Liege's wonderful fluff assassin of a Codex: Grey Knights.
Malal has a fortress in the Chaos wastes where he captures and trolls Greater Daemons by trapping them for all eternity unable to do whatever it is that they embody. This one Keeper of Secrets he has, for example, is caught in a field that nullifies all sensation so it can't indulge in cocaine fueled sex parties with Doomrider, thus eternally pissing of said daemon forever in the only way that works. He also put a Great Unclean One in a vat of Febreze; blinded and binded a Lord of Change and put him in a cage that never changes; and locked a Bloodthirster in a zen garden that cannot be destroyed.
As Malice, he still got his very own Chaos marine warband the Sons of Malice. Unlike his brother gods he is capable of entering the materium/Warhammer World through demonic possession if his followers had enough sacrifices, but since his followers rarely manage to not kill each other long enough to make headway towards any particular goal it mostly falls to his Champions to get shit done.
Zuvassin took on Malal's role in the canon, although since he's only been mentioned a few times he may as not exist either.
Back in the early days of Warhammer, GW's horribly retarded legal team lost the right to use Malal when the writers that came up with him, John Wagner and Alan Grant, left. Being notable for their previous work on Judge Dredd, the two had enough clout to insist that they own the copyright. Initially, they were going to replace him with "Malice", who is essentially the same guy but with enough changed so they couldn't be sued. They gave up on the idea however, relegating the whole concept of "Chaos fighting Chaos" to just shit the Chaos Gods do themselves (putting all four into "full retard" levels). This happened in the middle of a comic series by Brett Ewins used to introduce Malal featuring the Warhammer Fantasy champion Kaleb Daark who was going to assassinate one of the Chaos Gods of Order as a trial. Kaleb had a model previewed which was going to be released alongside Malal's Daemons, but thanks to the legal fuckery it was never released.
Getting Malal on the Table
As Games Workshop have proven themselves to be a bunch of IP abusing faggots, it once again falls to /tg/ to get anything done.
To field rapevikings of Malal in Warhammer Fantasy, run Warriors of Chaos models on Skaven bases. Nothing like blowing yourself AND the enemy up to say "It doesn't matter". Ironically (or appropriately depending on how you look at it) one of the best ways to represent 40k Malal dedicated Chaos Marines would be to use the Grey Knights codex. Think about it; elite marine units with sick powers and a plethora of daemon killing hardware, small army sizes, and more. Take an Unbound Dreadknight spam list and use Chaos bitz for a nasty Chaos smashing elite warband of Malal followers and lolstomp your opponents into the ground. Alternatively you can use C:CSM if you aren't a cheesed-out 12 year old or Daemonhunters vet.
Daemons of Malal
Have you ever lamented how you always wanted to put together a Malal themed army, but you didn't, because it wouldn't be worth it without the proper daemons? Well, now you no longer have a fucking excuse. Grown mightily indignant at Malal's lack of presence on the tabletop, several anons conspired together to bring you this. We even found models for it; they're at the bottom, listed as Hook Horrors. Play-testing would be very much apreciated.
Alternatively, modification to existing Daemon models using their original stats is fine too (a popular idea for example is a gleeful white and black Daemon Prince tearing itself in half along the color separation).
At one point in the 36th Millennium, the other four gods got really sick of Malal's constant interference in their plans. While Malal was at the time the strongest individual god, the other four knew that he couldn't deal with them all at once. So Tzeentch gathered the other gods and formulated a plan to deal with Malal once and for all. So one day Nurgle knocked on Malal's door and asked Malal to step outside, at this point Tzeentch signalled the others to attack and the other Chaos Gods jumped out from behind their cover and beat Malal to death. It required about a century of constant beating, but eventually Malal died. Upon his death the other Four Chaos gods celebrated for one thousand years now that the dick cheese was out of the way. Sadly Malal's bastard offspring Malice and Zuvassin managed to slip away. However, it might be possible that Malal/Malice may just be an Alias for the Emperor, working incognito, looking into ways he can ultimately end the four-fold scourge of the Warp. It might be possible he would use "Malal/Malice", using the powers of the Dark Gods against them. That includes using the Sons of Malice(Grey Knights?). Just as soon as Kaldor stops sniffin' Warp Dust. A final theory is that he's become a sneaky git and painted himself purple to wait out until the time is right to hit the big four just hard enough to finish them (considering the current state of Daemons on the tabletop, that might not be too far off), and in the meantime manipulates Orks into fighting each other by pretending to be Gork, then Mork (or Mork then Gork?) to draw them into arguments while causing the Imperium to weaken itself by BLAMMING everyone capable of uplifting the condition of man. He probably got the idea sometime after he left a giant sword for Farsight to eventually find, but before he altered Macha's biochemistry to amp up her hormone output. In fact, the truth is much more terrifying. Malal in fact managed to travel back through time to the present, where he is now trying to manifest. This is the source of the Zalgo legends across the internet.
In Warhammer Fantasy he still exists, and is secretly sponsoring Nagash's recent rise to power. He hasn't been seen or mentioned because he's been crashing on the Horned Rat's couch after burning his own house down.
Due to the fact that there is little concrete information about Malal content, especially in 40k where he has almost no presence, there are a number of ways to interpret the lore into a whole. Here are links to fan-created rule sets.
Dirges of Malal
Take the rot, to make it flesh.
Take the skull, the soul to rest.
Take their mind and give them peace.
Take their will. Sensations cease.
"We shall deny Nurgle their flesh to fester and rot"
"We shall deny Khorne their blood and skulls"
"We shall deny Tzeentch their destinies and fates"
"We shall deny Slaanesh their pleasure and pain"
"Death to the Dark Gods"
"For the Renegade God"
"Let the galaxy burn!"
To the Skin, Ice
To the Rot, Fire
To the Skull, Steel
To the Mind, Night
Followers of Malal
The Alpha Legion and their two primarch No such legion nor primarchs exist!
Tau assault units
Chris-chan's long-lost perfect, popular brother
Blood Angels units that aren't utterly overpowered
Assurances that don't leave Lord Bale cold
Vladimir Putin's estranged twin brother that isn't a complete badass
Irreligious Word Bearers
Ultramarines that deserve to live
Matt ward Fans
Chaos centurions and knights
Black Templar Librarians
Celibate and Sober Slaanesh Daemon
Cheap GW models
Asexual and Altruistic Dark Eldar
Ahzek Ahriman DOUBLEHERESY!*BLAM*
Justin Bieber's popular twin brother
Pacifist army ants
Commissars that don't *BLAM* their own troops.