[MaxWilson]

That's really odd. I've heard conflicting reports on whether casters will cast spells that take them over 200 fatigue (Graeme Dice insists they won't, others seem to have experienced it) but this sounds like a case where this is happening. S9 lets him use one extra gem, so he'd take 800/3 + encumbrance fatigue, which is about 240. If you added another booster to make him S10, he could use two gems for 800/5 + encumbrance, which would be 165 or thereabouts, which means he would cast it. Since you can't (IIRC) use another item booster without popping out the Eye of the Void, try casting Light of the Northern Star or Power of the Spheres on round 1, which will bring you up to S10 (and also give you +1 to penetration). He should then cast Master Enslave on round 2.

Let us know if that works.

-Max