I propose an new approach as follows:
POWER FROM PAIN
As Dark Eldar feed on the terror and agony of their foes, their own withered souls are invigorated and they become imbued with supernatural might, heightened aggression and a will of iron. In order to feast on the living, each Dark Eldar will want to twist the knife from as close a range as possible, though some enjoy the heady taste of death from friend or foe, and the most twisted of the Dark Kin get their energy from the smell of fear prior to a bloody end.
Power from Pain manifests for individual Dark Eldar units when certain criteria have been met. As they draw in the negative energies of the battle each unit may gain a powerful advantage as they synergise their potent physical prowess with their indomitable surging force of will.
The following PfP types will determine how and when each Dark Eldar unit can feel the Power from Pain
Blood and Souls – Death at close quarters yields the sweetest fruit. Each unit with this rule gains a single pain token if they are within 6” of a non-vehicle unit, friend or foe that is completely destroyed
(Most Dark Eldar will have this)
Nexus of Pain – In the fury of combat these brutal Dark Eldar feel the Power from Pain with a greater intensity. Each unit with this rule gains a single Pain Token if they are engaged in combat with an enemy unit and more than 5 wounds have been inflicted at the end of the assault phase upon either friend or foe
(Only the combat and elite Dark Eldar will have this as well as the above)
Feast of Fear – Certain twisted and evil Dark Eldar relish the taste of terror, growing more potent and nightmarish as the fear spreads. Each unit with this rule gains a single Pain Token if a unit, either friend or foe (excluding themselves) within 6” fails a Morale, Fear or Pinning test
(All Covenites will have this too)
Architect of Agony – The ancient Dark Eldar of Commorragh can only feed their withered souls when wholesale slaughter and destruction has been sown. The enemy must have died in droves, their champions reaped and their plans in ruins. Each unit with this rule gains a single Pain Token when the total of enemy dead reaches 20 AND in addition the enemy Warlord is dead
(Archon and Urien will have this)
Each unit may choose to hold their Pain Token which will provide the Adamantium Will special rule AND the following benefits:
Dark Eldar – Feel No Pain 6+, Fearless
Dark Eldar Coven – Fear, Fearless
Alternatively, each Dark Eldar unit may choose to expend their token for a powerful battlefield advantage detailed below. The way in which Synergies can be made differs depending on the type of unit.
POWER FROM PAIN SYNERGIES
Rejuvinate – Though they may be cut and broken, the energised Dark Eldar sees their wounds knit and injuries heal as they channel the stolen life energy into their body. At the end of the Dark Eldar Players turn the unit expends its Pain Token and each model in the unit gains the It Will Not Die rule (immediately making the roll)
Murderer – Enhanced with a heightened aggression, the empowered Dark Kin leap into combat with a fury and bloodlust that is nigh unstoppable. At the start of any Dark Eldar Assault Phase, a unit may expend a Pain Token to gain the Rage AND Furious Charge special rules for that Phase only
Shadow Stride – With charged muscles and enhanced celerity, the Dark Eldar move like fleeting shadows across the battlefield, redeploying in an instant. At the end of the Dark Eldar Movement Phase, a unit not currently embarked on a vehicle may expend a token to immediately make a free movement of D6” (Re-rollable with the Fleet special rule) with the move through cover special rule. This move can allow the unit to embark upon a transport if appropriate
Unstoppable – Even the most grievous injuries are ignored by the Empowered Eldar…they take the hit and continue to fight safe in the knowledge that their bodies will be repaired by the Haemonculi. The Dark Eldar unit can expend a Pain Token to gain the Eternal Warrior special rule for the remainder of that phase only
Nightmares Made Flesh – Both Covens freaks and lithe Wyches gain greater strength when deep in melee combat, their flesh cut and blood spilled. At the start of either players Assault Phase, excluding any where the unit has charged, the unit may expend a Pain Token and each model gains +1S for the duration of that Phase
So basics...each unit can only hold a max of 1 token, when its spent its gone but they can then earn more. The book keeping is minimal, a single token at any time with useful benefits earned through fluffy circumstances and spent with quick and potent results and no tracking or easily forgotten elements.
Master of Pain would allow a Haemonculus a re-roll to keep a Synergised Pain Token...and also for units nearby perhaps. Father of Pain the above but better. Additionally the Haemonculi could hold more than one Token themselves, allowing the Synergies and ongoing bonuses too