The person behind the hood was Jassin, the one who secretly fell ill on the first days of the journey.
It turned out he purposely poisoned himself to create doubt inside Arthur's heart, and inside the heart of everyone else present.
"Why did you do this? Answer me!" Arthur said.
He was yelling during the night, which made all the others wake up from their tents and come out to see exactly what was happening.
"Akhur, what happened?" Siora said.
Arthur already knew Siora and Jassin had that intimate moment, therefore he suspected she might have something to do with this.
He explained everything that had happened, and none of them could believe it.
"Jassin, is that true?" Siora said.
Jassin laughed maniacally, and said, "if only this person was not that strong. How did you notice my presence?"
Everyone knew now that Jassin was indeed the culprit behind the strange poisoning and now the murder attempt towards Arthur.