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I finished the game, I discovered 32 secrets out of 42 and 4 treasures out of 6, levels visited 26 of 27, the group was acquitted and Armand Rouquiner went to prison (red hair, black clothes, mine book and fake letter). I put the 4 essences on their pedestals, the only thing I got was an experience bonus, is there anything else related to the essences? Supposedly there is a level that I have not visited.
Regarding the fight against the dragon there is a bug that is to let it corner you on one side and the dragon does not attack you and you can kill it easily, it happened to me the first two times I fought against it by chance, the third time I beat it normally just going around the sides and killing the smaller units, until they stop coming out, then you can kill it normally 1 vs 1. There is also a book that can instantly kill an enemy in front of you, I used it against the dragon but it didn't work.
Regarding the fight against the dragon there is a bug that is to let it corner you on one side and the dragon does not attack you and you can kill it easily, it happened to me the first two times I fought against it by chance, the third time I beat it normally just going around the sides and killing the smaller units, until they stop coming out, then you can kill it normally 1 vs 1. There is also a book that can instantly kill an enemy in front of you, I used it against the dragon but it didn't work.