He knew he wasn't insane. He couldn't be. He knew the voice by name, even though the name had never been spoken to him. He simply knew the name without ever asking. And after doing some research, he was able to match the name he knew to a known supernatural entity. Saika. Eventual Shizaya.
EnniNova:
Про Дары смерти нам подробно рассказали в каноне. И вроде бы все понятно, все мы знаем. Но автор этой работы представил совершенно неожиданную версию истории Даров. И это очень интересно и страшно.