Creature of darkness: that am I.
Dare to harm me, I'll make you cry.
Cut short my life and before end of day,
Your family and friends: I'll make them pay.
Pure white my heart, brown is my dress.
Just who am I? Bet you can't guess.
(In reply to Answer
by K Sengupta)
In conformity with the first line, an onion grows underground. Accordingly, it can be deemed as a creature of darkness.
Cutting an onion indeed brings tear's to one's eyes. This satisfies the condition corresponding to the second line.
The third and fourth lines together indicate that consuming an onion inter alia causes bad breath.
We know that the most common variieties are the White onion and the brown onion. This satisfies the conditions corresponding to the fifth line.
Consequently, "I" referred to in the riddle is an onion.