Of course, he was insane. Anyone who'd lived in troll caverns for that long would be. Because his eyes were so sensitive to light, I had to speak with him in an almost totally dark room while a scribe outside took dictation.
He claimed to have been kidnapped as a small child then either captured or sold (his story changed several times as I reviewed it) to the troll clan. He grew up mostly among the trollkin, who seemed to take pity on him. He learned from them their fatalistic philosophy that is well documented elsewhere. Often during our conversation he made gestures that trollkin made.
Since the troll clan lived in Halikiv, it was natural that the subject of Arkat would come up. It was at this point that the startling revelation came up. He has been verified as telling no falsehood, and his knowledge of Darktongue has also been tested by Argan Argar acolytes and found to be good to excellent.
It is possible that since the source material was not written prose but recited poetry that there are mistakes in interpreting the true meaning of troll poetry. However, it is unlikely that there has been a change in the poem; the subject at one time heard the poem sung by a Mistress Race troll (this fact has also been verified).
The poem was, at first, the usual poem that is sung in praise of Arkat. However, a verse apparently not normally sung, was quite surprising. To summarize the verse, Arkat always was a troll. However, he could not be born a troll due to the Trollkin Curse. Therefore Kyger Litor, at the advice of the Spider, worked a trick -- Arkat was born a human instead. It was left to Kwaratch Kang to perform the ritual which was destined to work correctly: Arkat became, as the tales tell, a Mistress Race Troll.
If correct, this explains why, of all who have attempted the same transformation, Arkat was the only one for whom the transformation was complete. It worked because he was already a troll. The ritual merely had to change his outward appearance.
The poem goes on to explain that when Arkat confronted Gbaji, before Arkat could kill Gbaji, Gbaji struck Arkat such a blow that burned away the Darkness from Arkat. This caused Arkat to revert to being a human, and explains why Arkat was no longer a troll after the Gbaji War.
The subject was later found dead in his temporary quarters. No cause of death was established, nor was any Resurrection spell successful.