The answer of the eighth riddle - Diamonds And Illusions
Actually, you can just read the reviews, they look a lot more logical than mine XD But I will do it anyway(for the sake of my insanity), so here!
The first line "A letter from the dead was read"
» Someone read the letter, and name of the sender was a person that already dead.
The second line "Invited them to the house of Illusions."
» The letter told the readers(we didn't know who read it yet.) to come to a house/mansion of 'illusions'.
The third line "With diamonds, a cake, and poison"
» This line was obvious the line for keywords...
The fourth line "Made the creative, but simple cases occurred before their shocked eyes"
» Three things above was used to created the cases(plural), which at least one of them happened in front of people presented in the place(witnesses).
- The fifth line "Though the hidden message was easily resurfaced."
» There must be a dying message or something?
The sixth line "And the case was solved, by two brilliant minds."
» Two detectives solved the case...yeah
The seventh line "A secret confession was spoke out"
» Secret? probably love confession...who spoke it actually?
The eighth line "But unheard to a dense tan teenage boy"
» 'Tan teenage boy' was definitely Heiji, everyone knows that, right?
The ninth line "Who were teasing another boy about"
» Heiji like teasing who? Conan-kun obviously...
The tenth line "Certain zero confession of love."
» Indicated to Shinichi's confession to Ran in London "Love is 0"
Connect two and two together, the outcome was like this...
- The cases was the ones in "The Design of Poison and Mirage."(episode 652-655), in which Heiji got a letter(with the villa's key inside) from a dead man and old acquaintance of his mother who was murdered in the villa at Karuizawa.
- 'Diamonds', 'cake' and 'poison' referred to the murders tricks in the series. Diamonds was the "Crazy diamonds optical illusion" the wall and tiles in the villa's bathroom that the dying messages were hidden under. Cake = "Jastrow illusion", by made only one of others pieces seem larger(one that was applied a poison), though they were on the same size. Poison was also used for another murder involved the dictionary and "Gestaltzerfall phenomenon" made the victim to forgot how to write a certain word, open dictionary on certain page, lead to the victim's death.
- The one who spoke a confession was Kazuha at the end of ep.655, but Heiji didn't hear any of it because he was teasing Conan about his confession in London.
Short and Simple! Is the motto of these answer chapters! (Or maybe I'm just lazy, who know? XD)
Go! Before my weirdness can take over you! :P
(Wrote this chap when on the sugar high, so yeah XD)