Eliezer is in possession of a fact that he considers to be highly dangerous to anyone who knows it, and who does not have sufficient understanding of exotic decision theory to avoid being vulnerable to it. This is the original basilisk that drew LessWrong's attention to the idea. Whether he is right is disputed (but the disputation cannot take place here).

In HPMOR, he has fictionally presented another basilisk: Harry cannot tell some other wizards, including Dumbledore, about the true Patronus spell, because that knowledge would render them incapable of casting the Patronus at all, leaving them vulnerable to having their minds eaten by Dementors.

I know some basilisks, yes. Although, there is nothing I regard as actually dangerous. However, sharing things like this publicly is considered bad etiquette on LessWrong.

I know one.

Also I think you're missing the word "know"