Imp is almost de Facto pet for destruction warlocks.
The Imp's only method of attacking is through the use of its Firebolt. It uses lots of energy on this attack and won't use it again until it has the energy.
Make sure Firebolt is put on auto in the pet abilities bar: right-click to enable.
Make sure the target is not fire immune because the Imp will not attack.
The Imp's Firebolt also can be resisted, which definitely hurts the Imp. However, the Imp can also have its Firebolts crit to do extra damage.
A common mistake some warlocks make is to tell their Imp to attack when Firebolt isn't set to auto-cast. Since Firebolt is the Imp's only attack form, this gives it nothing to do; right-click on the Firebolt button to fix it. An Imp will also not attack if it is out of mana; it will also refuse to attack mobs that are immune to fire.
Originally, a quest chain was required to obtain an imp. After Patch 3.3.0, the imp became a trainable spell, purchasable at level 1 and requiring no quest chain. As of Patch 4.0.1, Warlocks will now start out with an imp minion.
As with the other warlock pets, the Imp talks. When asked to attack, it will sometimes complain about it in Common or Orcish. At other times, it says something indecipherable in Demonic.
Before the patch that gave the Imp voices here are some of the phrases the Imp used to spam in the chat menu.
"What? You mean you can't kill this one by yourself?"
"Make yourself useful and help me out on this one!"
"Release me already, I've had enough!"
"Alright, I'm on it! Stop yelling!"
"No shi rakir no tiros kamil re lok ante refir shi rakir"
"Maz ruk X rikk xi laz enkil parn zila zilthuras karkun thorje kar x zennshi"
Imps have an idle animation of capering about, sometimes performing a front flip as well. They never seem to stand totally still and often fidget.
While swimming the Imp performs the doggy paddle.
Once an imp-type enemy holding a weapon dies (for example: Vile Familiar), you can see his thumb twitch.
The imp has no voice overs when summoned.
Voice overs when ordered to attack:
"Is this REALLY necessary?!"
"This was NOT in my contract!"
"Can't we all just get along?"
"Ohhhh sure, send the little guy!"
Voice overs when ordered to cast a spell:
"What's in it for me?"
"Do I have to?!"
"Ahh! Okay, okay, okay, okay, okay, okay!"
"Yeah, I'll get right on it."
Voice overs when dismissed:
"You know, we've had a lot of fun together, it's been really special, but I think it's time I should start seeing other warlocks. Just a little on the side. No no no it's not you, it's not you, it's me. I just need my space, it's nobody's fault."
"Don't call on me, I'll call on you."
"Argh! I feel so used!"
"*indistinct grumbling*...I wish...*indistinct grumbling*...wish you were DEAD."
Patch 6.2.0 (23-Jun-2015): Some Warlock pets received a visual upgrade and are now using higher-fidelity models.
Patch 5.2.0 (5-Mar-2013): The Imp ability Blood Pact has been removed.
/Patch 4.0.1 (12-Oct-2010): Warlocks now start with an Imp at level 1.
Patch 3.3.0 (08-Dec-2009): This ability is now available from the trainer for level 1 warlocks and no longer requires a quest to learn.
/Patch 3.0.2 (14-Oct-2008): Health increased by 20% , Armor increased 16%, mana regeneration increased 200%.
Patch 2.0.3 (09-Jan-2007): Warlock pet "Imp" will now correctly receive their spell damage bonus.