What is the meaning of invoker level for a Warlock?

Hellfire Warlock prestige class (Fiendish Codex II: Tyrants of the Nine Hells variant, p. 89) mentions:

Invoking: At each level, you gain new invocations known, increased damage with eldritch blast, and an increase in invoker level as if you had also gained a level in the warlock class. You do not, however, gain any other benefit a character of that class would have gained.

What does the book mean by invoker level? The term invoker level is not mentioned in Complete Arcane under Warlock class.

Heavy thrown implement for Invoker

I have this idea of a hybrid Fighter|Invoker, using Cleave and Deft Hurler to throw Divine Bolts. Deft Hurler (Dr 368) lets you use a ranged basic attack after Cleave:

Cleave: You can forgo dealing damage to the adjacent enemy to instead make a ranged basic attack with a heavy thrown weapon against one creature other than the target of your cleave. This ranged basic attack does not provoke opportunity attacks.

RAW, implement powers used with weapon implements still count as “with a weapon”.

Power of Skill (DP) turns Divine Bolts into a basic attack:

You can use any power you have that is associated with this feat as a basic attack. (Divine Bolts is listed)

Now I just need a heavy thrown weapon with the highest damage that is usable as an implement by an Invoker.
Bonus points if it is possible without a multiclass feat, as I want to use Seeker MC in paragon to get Primal Eye.

If it matters:
Str 17, Con 12, Dex 13, Int 8, Wis 16, Cha 8
Goliath (but any Str/Wis race will do)