This particular coven operates as a powerful and wide spanning enterprise focused on accumulating power to direct the world in ways that align with their interests. Witches gain more power through a dark ritual, which steals the life force from an unwilling victim and adds it to their own, making their magic stronger. Their physical forms also evolve, becoming higher life forms in the process. Rituals are usually performed in groups, usually of about 7 members of a coven, as the ritual is dangerous to perform and more people participating lessens the risks, with the drawback being that each witch gains a percentage of the power.
say a victim has a power level of 50. Each of the 7 people will receive a portion of that divided equally.

Attached to each coven are soldiers or bodyguards. These carry out the tasks that require brute force or violence, such as war, or defending the coven from outside enemies. They also guard the witches while dark rituals are being performed. A coven of small amount of witches can have dozens or hundreds of soldiers protecting them, with bigger covens having more than smaller ones. These soldiers also grow in power as the witches in the coven increase their own. Their physical forms also change the stronger they get, becoming something other than human. The problem is how this is supposed to happen.

These soldiers don't have access to magic, and therefore can't participate in these rituals, yet I want them to become stronger, faster, and all around better fighters due to the groups in a coven performing them. However, covens cutting them in on the intake would deplete the sharing of power among their members even more than it already has, since the amount of energy one gains would be less the more it is spread out. With a large amount of soldiers all absorbing equal amounts of energy, the power gained eventually becomes negligible.

How can I get around this problem without making witches weaker?

First, let me try to describe the limitations as I've understood them. It seems that the energy can be taken from the living things only - there's no 'background' magic of the world to tap into, no ley lines, no spiritual patrons granting power.

Second, it seems that the magic itself is net-negative. The amount of power gotten through the ritual is smaller then there was power originally.

If any of that is wrong, then you have simple ways of granting power to your soldiers - a ritual involving a ley line, or a huge ritual with all of them participating that gets into weird amplification effects just due to the number of the participants involved.

If my understanding is true, though, I see only one solution at the moment - the rituals used on the soldiers may use animal, not human donors. There may be a number of issues why witches do not absorb the power of animals into themselves, both practical and cultural. Animal power may change the behaviour and reactions to be nearer to the donor animals, or they may plainly think themselves too good to use that. For the soldiers, however - what witch won't like a dog-obedient bodyguard with improved strength, speed, senses of smell and hearing?

Separate the wheat from the chaff.

Not every particle of magic energy is created equal, there are impurities which can still be used for power, just not magical power. However that energy has to go somewhere or else it just makes a big mess everywhere and makes it harder to pick out the really powerful stuff.

Ages ago witches would just channel this "slag" into a familiar or a slave. Until they realized the effect this energy had was to increase aggression and physical strength. As well as other transformations.

That's also around the time that the wars broke out and bodyguards became necessary for rituals. Strengthening your own bodyguards with power that would make them super human was a no-brainer. It just meant they needed extra loyalty spells to be cast on them in order to ensure the added aggression was not directed toward their masters.

Which isn't too much of a problem with some. Certain people are predisposed to being addicted to power, and energy can drain if you don't participate in rituals so they have to remain loyal to the coven if they want to get their "fix".

Look for people who have had power taken from them as children, they had parents that exerted constant controlling abuse over them or maybe their parents were killed, or their parents were oppressed by a dictatorial overlord. Look for the people who experienced these things and say "I won't ever let myself become powerless". Look for the anger burning in their eyes when they say this.

Those people are the ones who are perfect to manipulate into becoming dependent upon the power you give them. You can convince them that they can achieve whatever their heart desires, if they just follow your kind hearted guidance.

Your witches can charge some sort of "magic batteries", which are then used by the non-magical soldiers as power enhancer.

In this way the witches can store the surplus magic produced during times of lower usage, and the soldiers are kept dependent from the witches to get their boost. You know, trusting is fine, not trusting is better.

"Physical Evolution" is catalyzed by the ritual, not powered by it.

In your question you state that your ritual has two effects: first to make their magic stronger, and second to evolve their physical forms.

1. Since the soldiers are non magical, the magical power from the ritual isn't shared among them.


2. Anyone who is in the vicinity of the ritual will be exposed to a type of radiation which allows, or catalyzes, their cells to mutate and divide causing physical evolution. The energy needed for this evolution is taken from the bodies of the individual, fat reserves will be drained, the soldiers and witches will be very hungry afterward (and if they're wise they'll have a large feast prior to the ritual to get the most out of it).

Note that the idea of catalysis is that the presence of something reduces the amount of energy required to get something started and as a result speeds up the process (often a process that wouldn't happen otherwise). But the energy comes from somewhere else, but since this isn't magical energy it is physical energy, it's perfectly reasonable for it to just come from the food that the soldiers eat.

If you are a witch performing dark rituals and such you really REALLY don't want your bodyguard super-soldiers being able to turn on you.

The easy solution to that problem is soulbind or otherwise magically bind your soldiers to you in a way that makes it impossible for them to go against you. (This could go from straight up inability to not follow your orders to simply linking their life to yours).

However, creating this link between your soldiers and you (of course done in another dark ritual) has another effect: while the soldiers don't have magic themselves, the magic of the witch who bound them enhances the soldiers strength.
This is a passive effect of the bond and not directly caused by the dark rituals that increase the witches magic.