Any Man Advice

[Manuk]

if you are a slave you don't cast spells.
but if you are a master your skill can be increased so you meet the requirements right?
or there is some hidden mechanism?

[DrPraetorious]

There is a hidden mechanism which I described above.

Say I'm an S4 caster with two S2 communion slaves, and I cast "Will of the Fates" (S4, 400 fatigue.)

I'm boosted to S5, so I pay (400 / 2) / 3 ~= 66 fatigue.

My slaves are boosted to S3, so they each pay (400 * 2) / 3 ~= 266 fatigue, and drop dead.

I'm not sure how spending extra gems works into this - it may or may not reduce the fatigue spent by the slaves.

[Manuk]

So, at least 2 slaves for a 400 fatigue spell so they don't drop dead.
Because it makes no sense to me to have slaves of lvl 5 in any magic.