The only way i can imagine the effect of such a drug is like this: It activates parts of the predator brain in such a way that the conscious is separated from the subconscious and so the predator, during the ritual, would have two different personalities. One on the outside and one on the inside. To the outside world, the predator becomes extremely violent and no longer able to distinguish between pain, pleasure, friend or foe, thus exhibiting the symptomes of "demonic posession" (allegedly, he is being possessed by Cetanu himself).
On the inside, however, the predator find himself in a dream world where it is his quest to defeat the God of Death. Of course, the only way it would be possible to have a certain specific hallucination at this time would be if the predator had been raised since birth to focus on this one most important battle before becoming a "man". Therefore, before taking the drug he would meditate for weeks on this upcoming battle in order to train the subconscious to expect it so that the hallucination is correct. This all would be kept as very strict and important tradition.
During the ritual, the predator would exhibit unnatural strength and resillience, therefore he would be extremely hard to restrain. This would require especially trained predators to guard him, continuously provoking him and defending against his attacks so that he has something to punch at and doesn't harm himself in stead. Of course, there would be a time limit (a few days maybe) after which the predator's mind would be completely lost, signifying that he lost the inner battle with Cetanu, at which point the Elder would have to kill him.