Lead by 'The Apex', once the chapters Chief Shaman (Librarian and Apothacary). This beings two top lieutenants are both sourcerers of great power. Despite their struggles to gain 'perfection', the chapter is extremely well organised. Ascension up the ranks comes only when the marines psychic potential, martial prowess and physical appearance have increased from their peers.
They recruit from the surrounding plains, by way of forcing aspirants through a terrible maze of psychic trickery, daemonic corruption and gene seed implantation by supernatural means before they reach the fortress monastery.
Their typical method of combat involves use of many infantry to form a solid line of bolter and heavy weapon fire. The enemy is then assaulted from the side by possessed marines, veteran troopers or jetpack armed warriors.

There is the basic fluff out of the way. Here is the basic rules that i was looking at for them. I wanted to have some aspect of psychic power that could be applied to each of the marines, but without having troops with invunerable saves and without it being unflavoured.
Note, i havent decided on points cost or if they are entirely appropriate yet.

Apex Gods Special Rules

The Apex gods use a number of special rules, indicated here.
“We are Perfection..”
The apex gods strive for perfection in everything they do. Their worship of slaanesh is an extension of this. All models may reroll failed leadership and pinning tests they are required to take (this doesn’t include psychic tests).

New Wargear/upgrades

Eldar spirit stone: A spirit stone, ripped from the armour of a dead eldar warrior. This allows the unit to ignore a single Perils of the Warp result.

Stolen spirit stone: The spirit stone of an eldar warrior, captured while the warrior was still alive and allows the user to place a daemonic spirit or part of their own conciousness within. Adds D6" to the range of psychic powers (note: doesnt allow non ranged psychic powers to be used on models other than the user).

Warp technology: The unit is equipped with a form of warp based technology that empowers the user with blasphemous energies, either by purpose or as a side effect. When reduced to zero wounds, place the model on its side. At the end of the current phase, roll a D6. On a roll of 6, the model may stand back up. This may not be used if the model was killed by a close combat weapon that ignores armour saves, or a weapon that causes instant death.

Master crafted weaponry: May allow a single reroll to hit per turn.

Icon of chaos glory: The icon of chaos glory add +1 to the units leadership while the icon is present.

Latent powers: The model/squad has latent psychic potential. Any model with this upgrade may roll D6 on the following table at the start of the game, before deployment:
1: The models psychic powers are dangerous and uncontrolled. All psychic powers (friend or foe) used within 12” are nullified on a 4+. If the roll is 1-3, 1 model takes a wound, no saves of any kind allowed.
2: The models all have telepathy. Unit coherency is 3” for this squad, independant characters may join squads within 3”.
3: They permanently cast an illusion of power. All models are at -1 leadership in CC with them.
4: Time distortion: when testing for sweeping advances, the model rolls an extra D6 and picks the highest.
5: ESP: any unit fired at has -1 from its cover save, to a minimum of 6+.
6: Enhanced bodies: the unit may ignore D3 failed wounds per turn (roll at the start of each turn), so long as they don’t ignore armour saves, are ap1 or 2, or are double the models toughness.

Minor powers: Many in the Apex gods have psychic powers that have manifested themselves. These are bought at the cost listed within each units entry. Each turn, a psychic test must be taken in order to use it during that turn, although it doesn’t count towards the total number of psychic powers used by that model that turn, and the powers don’t have to be used if you don’t want to:
Psychic shell: The psyker has a field of distorting warp energy that soaks up and deflects the energy of shots fired at him, although sometimes the strain may be too much. The model has a +1 to his invulnerable save (or a 6+ if he hasn’t got one). If a 1 is rolled and the model isn’t slain by the shot it failed, then roll another D6. On a roll of 1 or 2, he takes another wound.
Warp strength: The psyker can make himself stronger by dragging power from the warp. He is at +1 strength
Necromantic: The psyker can keep the bodies of his fellows alive even with terrible wounds. Any models within 3” may attack back during a combat even if killed before his initiative is called. This doesn’t apply to any weapons that ignore armour saves or causes instant death.
Accelerate bullets: Any rolls of 6 to hit with shooting count as being 1 point of strength higher. If any of the rolls are 1, the model automatically counts as rolling 1s for all its to hit rolls.

The apex gods use the following units from Codex: Chaos space marines with the following changes:
Chaos sourcerer
Chaos lord
0-1: Daemon prince
0-1:Greater Daemon (counts as part of Force organisational chart)

Noise marines
The Beautiful (counts as possessed except costs 28pts and daemonkin is replaced with Rending claws)
Chaos Dreadnought

Chaos space marines

Fast attack:
Chaos bikers
Chaos raptors
The Ugly (Spawn)

Heavy support
Chaos Havocs
0-1: obliterator cult
Chaos land raider