Castle Ravenloft - Hunt For Strad´s Part 1 Question

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Im playing the adventure and I got confused by the Strahd tactics.  Do I have to bring him to 5 hit points so he transforms into myst and goes away or I have to take all 12 hit points to kill him?
Also, it says that when he haves 5 hit points or less you have to place a new monster. Does it means that he does not attack anymore and we just keeping placing new monster untill weré able to kill him?
We are playing it right now so if anyone can help us now, we would be very happy Cool