I have the rules for it printed out in my folder that I bring with me, and keep meaning to leave down there.
basically it works as follows. When a mutant or magister levels up and rolls a new skill result, you may instead send them to the pit to request the shadowlords favor. You roll 2d6 and consult the chart that follows
2: The character is eaten by crazed deamons living in the pit
3-6: Your prayers are ignored, the character gets nothing from this trip. (Basically you get nothing for this roll and as such get nothing for that level. But at least your guy is still alive)
7-8: Free mutation, you roll another d6 on a 1 your character losses one point from one of its characteristics. (Ex. loss one point of Speed or Strength, your choice.) If you roll a 2-5 you may pick a free mutation to give the character from the mutations list.
9-10: Chaos Armor, the character's skin becomes like armor giving it a 4+ armor save that can be combined with shields and does not affect the character's ability to move or cast spells.
11: Demon Weapon, your pick the way the weapon looks but it does not have any of the abilities of the type of weapon it just looks like it. This weapon gives the user +1 Strength and +1 to hit in close combat.
12: Possessed. the character becomes twisted by chaos, gaining +1 Weapon Skill, +1 Strength, +1 Attacks, and +1 wounds. This does not count towards the models racial maximum in those stats, also the model losses D3 skills and may no longer use weapons or armor. Except the model may use deamon weapons and chaos armor.
The benifits can be great. Would I risk a good magister on them? Probably not when you can get a new spell or lower the difficulty of one you already have. As for mutants, well early on these are great but if one gets really good and then dies in the pit you'll be kickin your own ass for that one.