Increases the damage done by your Shadow Bolt spell by X%, and your Shadow Bolt causes your target to be vulnerable to spell damage, increasing spell critical strike chance against that target by Y%. Effect lasts 30 sec.
Improved Shadow Bolt is a talent in the Destruction tree, you will need to spend 5 points in Destruction to max it.
The debuff applied to target is a magic effect called Shadow Mastery, which should not be confused with the Shadow Mastery talent found at tier 6 of the Affliction tree.
The Shadow Mastery debuff and the mage Improved Scorch debuff do not stack. The Shadow Mastery debuff takes precedence over the Improved Scorch, and will override Improved Scorch, however the opposite is not true.
As a result of the priority of Shadow Mastery over Improved Scorch, if a talented Warlock and Mage are in the same group, it is better for the warlock to cast Improved Shadow Bolt than for a mage to try to maintain the scorch debuff, as scorch is considered a DPS loss within the mage's rotation .
Patch 3.1.0 (14-Apr-2009): Now increases the damage done by your Shadow Bolt spell by 1/2/3/4/5%, and causes your target to be vulnerable to spell damage, increasing spell critical strike chance against that target by 1/2/3/4/5%. Effect lasts 30 sec.
Patch 3.0.8 (2009-01-20): Changed from applying a debuff to the target to a self-buff on the Warlock.
Patch 1.11.0 (20-Jun-2006): Periodic damage spells and non-damaging spells will no longer consume charges of Shadow Vulnerability.
Patch 1.10.2 (02-May-2006): Will now proc properly after landing a critical strike with Shadow Bolt.
Patch 1.2.1 (21-Dec-2004): Added a limited duration to the effect.
Patch 1.1.0 (07-Nov-2004): Changed from increases the next 1/2/3/4/5 sources of Shadow damage dealt to 4 sources.