Hi Tziouv,

I know this answer comes very late, but I just discovered this Forum and signed up here. I hope I still can help you ;)

Once the Chaos Dragon reaches a certain low of HP (something between 2-5 % of his maximum live, i think) the battle goes on for a certain amount of time (3 seconds or so) and then ends.
Then there are two possibilities: Either you bring his HP to zero within this time, which means that he dies, or, well, you don't. If the second thing happens, the dragon changes back into the old man telling you that your powers are formidable and that stuff and then go to the portal. While he goes to the Portal, you have one more last chance to attack and kill him before he reaches it. If you don't do so, he leaves.

So effectively you either kill the old man (either while he still is a dragon or while he fleas) or let him escape. There are no big differences between the two endings, it's only that if you kill him the words of Galileo and DaVinci will be friendlier and also your spirit will appear and congratulate you, which he does not if you let the old man escape.

Hope I could help you ;)

Btw, question back^^ I just had a character with negative karma finishing the game for the first time and my spirit was the one for tribal magic (don't know his name in English). I ran through all the variations of the evil ending (diplomacy, kill old man, let old man escape, let DaVinci/Galileo die and not etc.) but in none of the variations my spirit appeared to talk to me. Does anyone know whether the other two spirits appear in one of the evil endings if you chose them as your spirit? Or do all of them hate you so much for being evil that none of them would give any comment? :D