Toshiirx
Darkness.
According to Wikipedia, it is the polar opposite to brightness, an absence of visible light. It is also the appearance of black in a coloured space. The emotional response to darkness has generated metaphorical usages of the term in many cultures.
Although, I don't think that that paragraph truly sums it up.
I know that darkness can engulf you.
Take you over completely.
Each human has a specific amount of darkness within themselves, and if it takes you over, there is no return. You become a monster. An abomination. You must keep it in check, lest you lose your sanity.