OK. fgetc() returns a single char from the file. It does not return a string, and you cannot cast a char to a char*. Casts are tricky business, and you shouldn't use them unless you know EXACTLY what you're doing. If the compiler complains then there's a good chance that a cast won't fix the problem, or will create a new problem or make the problem worse (in this case, it changes a compiler error to a program crash).
ByteFromText seems OK, but what you need to do is read 8 characters from the file before calling it. Put those characters into a temporary buffer that is at least 9 bytes long, add a NULL terminator (a zero byte), and call ByteFromText with the buffer.