Break this class

[Biffoniacus_Furiou]

Yeah, undead are not taken into account. But Abjurant Champion and Eldritch Knight? They don't advance invocations or any of the magister's other special abilities.

From Complete Arcane page 18, under "Warlocks and Prestige Classes"

Warlocks benefit in a specific way from prestige classes that
have “+1 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 increased caster level when using his invocations 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 his eldritch blast
damage (treat his combined caster level as his warlock class
level when looking at Table 1–1: The Warlock to determine
eldritch blast damage) and his eldritch blast caster level (half
his total caster level from his warlock levels and his levels
in the prestige class that grant him an increased spellcasting
level). A warlock also gains new invocations known at
these prestige class levels as though he had gained a level
in the warlock class.

I was assuming that any "+1 level of existing class" prestige class would advance both spellcasting and invocations/EB. If this is not the case, then you should specify that in the class feature descriptions, because currently you include "like Warlocks" which would make it work as I described. There's also the Legacy Champion prestige class in Weapons of Legacy, which adds 8/10 levels of existing class features, which would make your last ten levels grant 3/4 BAB, d8 HP, and 4 skill points/level. Plus at level 22 you'd have Magister class features as though you were level 20 in the class, and it would continue on from there.