18 is an important number for dark occultists because it is equal to 6+6+6.
18 is also equivalent to the number 9 since 1+8=9.
For the significance of the number 9 see The Occult Significance of 9.
It strikes me that 18 is in many countries the age of majority, i.e. adulthood, and it could be asked whether this is an accident or, given the occult hand in so many aspects of everyday life, a deliberate choice.
I also recall a rather poor Moby album called 18, not that that necessarily has occult significance.