Detective Conan

Heiji receives a letter from Wakamatsu, the preseident of a design company and an aquaintance of his (Heijis) mother.
The content of the letter was from the murderer who killed Wakamatsu.
He wants to meet Heiji and confess his crime, but 'there's the problem of being seen at your home. I am a warped existance made by mankind ...
I'm an illusion that cannot be seen fair and square ... If you want to meet, it will have to be on the night of the next full moon. I will wait at the house of the killed man.'
At the end of the letter there's Wakamatsu's address and the key of his house.
Kazuha went to that house with her friends and saw a creepy person with long hair in the bathtub, but suddenly the light went on and off and then a piece of paper with the words
What disappeared was merely my flesh ... do not mind the letters (of the alphabet)" was in the bathtub.
Next to the person sitting on the wall of the bathtub "EYE" had been engraved, but that,too has disappeared.
Wakamatsu was also stabbed to death in the bathtub. There had been blood on the tip of his fingers, thus a dying message should have been there, but it had disappeared.
(It could be seen that a tile had been switched, so it was obvious how the message had disappeared.)
The five of them go to Wakamatsu's house. (Sera also wanted to go, but Heiji told Conan to be smart and refuse).At Wakamatsu's house there are his wife and son, the managing director, the chief designer, the secretary and a maid.
The son dies after eating the baamukuuhen (?) that the managing director brought. The chief had tasted it first and said he didn't need it.
The maid, who was the first to discover the victim cut the baamukuuhen. The secretary had distributed the black tea to everyone.