[Plane]

Another possible anti-exploit thing I just thought of (and relative simple to implement) would be when using IQ for the energy formula, it could be "current effective IQ" rather than "maximum IQ".

This would for example, involve shock reducing energy output for creatures lacking High Pain Threshold. If they died from a 4 dmg attack then -4 to IQ would mean no energy at all from creatures with IQ 1-4 and a mere 1x multiplier from IQ 5 sacrifices.

You could avoid that by using a series of smaller attacks (the final blow that kills them does just 1 damage, only -1 to IQ) but that could make the timing of the spells culmination a lot trickier since 4 HP will more reliably cause a Death Check for an animal than 1 HP will.

Also if we inverted the High HP and Shock rules rules for Low HP creaturs (ie if you are -1 to IQ per 2injury with max HP 20 then you could be -2 to IQ per 1 injury for HP 5) then this would further reduce the IQ multiplier for low-HP creatures. IE an IQ 4 HP 5 dog would only have a 2x multiplier (not 3x per applying usual shock) to the lower of HT/HP.

This could also apply to stuff like temporarily lowering IQ through spells like Drunkenness (M136) so if you used that to lure a potential sacrifice and tie them up, you can't keep them drunk (too stupid to escape) during your ritual, you'd need to end the spell to get maximum output of the sacrifice.

That could either be at the culmination (in which case it's not particularly limiting) or maybe it's whatever the lowest effective IQ was at any time during the 60 minutes of casting? So if during the casting the roof collapsed on your intended and he suffered -4 IQ from shock you'd probably want to restart that ritual and keep them at full IQ throughout it (after reinforcing the roof)

Another way this could reduce output is if using the optional Skull Wounds table (MA138) since that can cause a permanent IQ penalty (as can a stroke via the Neck Wounds Table) so you have a random chance of reducing your output if you kill them in a single major wound instead of gradually.

The way that High Pain Threshold would insulate at least against the 'shock during the sacrificial stab' also creates an opportunity to put the Spirit Slave ritual (Thaumatology 162) to work, since it imparts HPT. If you have access to both magic systems (SS:S is traditional magery, path of spirit rituals are Path/Book magery) then you'd probably be doing that a lot regardless since it makes them easier to entrap via other mind-manipulating magics (traditionally that -10 to will was for all purposes in 3e but 4e changed it to spells only)

Maybe to offset that, Hidebound guys should be considered -2 to effective IQ? If you have an anti-talent or incompetence for a bunch of IQ skills (art/engineering) then it begs the question if you're really enjoying the effects of an uncompromising IQ point...

Slave Mentality could also be considered... if you have IQ-8 rolls to take voluntary actions maybe you're -8 to IQ for the Sacrifice formula?

If also using the "lesser of IQ or Will" idea then the -10 to Will could also be used to prevent Spirit Slave exploitation (ie you would need to have someone who can resist you with at least Will 1 to get a multiplier)

Spirit Slave already gives it's own independent reward (immunity to spells cast by those not holding the soul jar, possible free ghost servant) so lacking Synergy with sacrifice wouldn't be horrible. If you really needed max sacrifice energy more than a ghostly servant then you could just terminate the effects of the Spirit Slave ritual just before culminating (or initiating, depending on approach) the Sacrifice spell.

Spirit Slave probably doesn't even need that "potential ghostly servant" perk. If you just let the -10 to will apply to them in the afterlife then the standard spells for manipulating ghosts (Bind / Exorcise / Lay to Rest / Summon) would be hard for them to resist. Especially since right after dying a ghost is probably effectively stunned / unconscious (ie if assigning a new char template keep their HP depleted so it takes a bit of time to shake out of the deathly stupor...)

What you could probably do is cast these rituals in advance (before even killing them) and just have them delay in going off until the point of death when they actually become valid targets.

There's probably even ways to use spirit's FP to engineer their own enslavement, like using the aforementioned necro-spell to steal it's FP to use for maintaining your Mind Control.