Description
Your 
[Shadow Bolt] critical strikes increase Shadow damage dealt to the target by 4%/8%/12%/16%/20% until 4 non-periodic damage sources are applied. Effect lasts a maximum of 12 sec.
A common misconception is that this effect does not apply to DoTs. In fact, it has full effect, however, DoTs do not consume charges. It is an excellent talent for the PvE environment, particularly in a raid scenario as all raid members using shadow spells will gain benefit from it. It is also more beneficial for PvP than its counterpart, Cataclysm.
Direct damage shadow spells from priests and from wands with shadow damage consume a charge of the debuff Improved Shadow Bolt as well.
Each swing of Succubus (Warlock pet)'s spell: 
[Lash of Pain] also consumes 1 charge of the debuff. Please take this into consideration when sending pet in auto attack.
See Also Formulas:Shadow Bolt