A new blog entry by Incubator Games creative lead Radek Koncewicz on Gamasutra eloquently covers the intricacies involved with studying the eight circles of Zerthimon with the help of our Githzerai companion, Dak'kon, in Planescape: Torment. Radek goes on to explain why unlocking the secrets of all eight circles was "one of the most moving moments he has ever experienced in a videogame":
With the sixth layer, both TNO and Dak'kon receive a new spell. To unlock the seventh and eighth layers, TNO's intelligence must be high enough to work the mechanism, and his wisdom high enough to understand the lessons themselves. Great memories. Thanks, LadyVox.
This is a nice transition of student-to-teacher, and ultimately rewards Dak'kon with some permanent stat increases. These in turn affect the karach blade, empowering it with each increment.
The lessons of the Circle also lead to the truth behind Dak'kon's and TNO's past.
The ruthless "practical" incarnation originally found Dak'kon close to death in the world of Limbo. He desired the karach blade, so he ensnared the gith in a devious trap. By constructing the Unbroken Circle of Zerthimon and speaking of its lessons, he showed Dak'kon a glimmer of hope to his spiritual ailment. In exchange, Dak'kon promised to follow TNO until his death, effectively becoming bound to the immortal for all time.
This enslavement constituted the greatest sacrilege for the zerth, yet it was the only salve for Dak'kon's moribund soul. By completing the Circle, we finally brought him the resolution he so desperately craved.