Use Hellfire
Ingredients:
6 levels of Warlock
3 levels of Ruathar
3 levels of Hellfire Warlock
8 levels of whatever other full casting PrC you want
Feats: Look them up here
Note: It is debated if the Strongheart Vest trick works with Hellfire Warlock's abilities. If your DM thinks it doesn't, take a level or two in the Binder class from Tome of Magic first before going into Warlock and bind Naeberus to heal 1 point of ability damage per round.
Objection!Ruather will not help you with warlock levels.
The +1 is to spellcasting levels not CL so it won't aid in any spell penetration checks for your EB and a warlock is not a spell casting class (see Complete Arcane)
[/sblock]Complete Arcane said:Warlocks benefit in a specific way from prestige classes that have "+1 spellcasting level of existing arcane spellcasting class" or "+1 level of existing spellcasting class" as a level advancement benefit. A warlock taking levels in such a prestige class does not gain any of his class abilities, but he does gain an increase in caster level when using his invocation and increased damage with his eldritch blast. Levels of prestige classes that provide +1 level of spellcasting effectively stack with the warlock's level to determine eldritch blast damage (treat his combined class level as his warlock level when looking at table 1-1: The Warlock) to determine eldritch blast damageand eldritch blast caster level (half his total class level from his warlock and the prestige class that grants him an increased spellcasting level).[Errata'd out.] A warlock also gains new invocations known at these prestige class levels as though he had gained a level in the warlock class.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.