Beyond of Evil: The culprit is maybe not a huge surprise but it is not the one you first are meant to believe either. So there is a mystery that goes on for several episodes. I do have to say though that I have mixed feelings about the drama. I did find the middle part to be very stretched, so if you didn't find the beginning interesting then you maybe will lose interest in the middle all over. The ending is well done though.
Trinity of Shadow - this is one of my top 4 dramas. It is so well done and has such a shocking twist (at least it was for me the case). I really really loved it but yeah I get what you mean. it was for me a little hard to get into at first too. But it really is a gem, especially when from the middle to the end onwards. It questions a lot of morals. And you really only learn all the puzzle pieces in the final episode.