Wikia

Empowered Imp
Your Imp's Firebolt has a X% chance to cause your next Soul Fire spell to be instant cast within 8 sec.
Usable by
Class	Warlock
Properties
Location	Destruction, Tier 6
Affects	Imp
Ranks	2
Requirements
Points required	25 + 2 points in [Burning Embers]
Spec specific	Yes
Related buff
Magic Empowered Imp Your Imp's Firebolt has a X% chance to cause your next Soul Fire spell to be instant cast within 8 sec. Duration: 8 seconds

Empowered Imp is a talent in the Destruction tree, you will need to spend 43 points in Destruction to max it.

Rank table

Rank	Chance of Instant Soul Fire
1	2%
2	4%

Prior to Cataclysm, the chance of crit increase was for the Warlock, and not for the Imp.

Patch 4.0.3 (15-Nov-2010): Effect changed from "Increases the damage done by your Imp by 10-30%, and all critical hits done by your Imp have a 33-100% chance to increase your spell critical hit chance for your next spell by 100%. This effect lasts 8 sec." to "Your Imp's Firebolt has a 2-4% chance to cause your next Soul Fire spell to be instant cast within 8 sec." Ranks reduced to 2 from 3. Points required reduced from 40 to 25, but requires 2 points in [Burning Embers].
Patch 3.3.2 (2010-01-02): The pet bonus damage provided by this talent has been increased to 10/20/30%, up from 5/10/15%.
Patch 3.2.0 (04-Aug-2009): The warlock's critical hit chance is increased to 100%, up from 20% for the next spell cast after the Imp critically hits a target.