I just want to make sure I'm reading this properly: According to the Diabolist information in Princes of Darkness, the class level for determining Imp's "animal companion-ness" (if you will) is the Diabolist level plus "the highest caster level."
So by my reading, if one has other prestige classes that add to the base class CL, they all stack for this. Specifically:
Wizard 5, Diabolist 1, Hellknight Signifer 5: Cast as level 11 wizard, so level 11 for the purposes of talking about this Imp.
Wizard 5, Diabolist 1, Hellknight Signifer 3, Cyphermage 3: Same, but total level 12.
Right?
from the wording wouldnt it put it above your character level? since diabolist gives caster levels? seems like a 5 wizard 5 diabolist should have a lvl 15 imp, since your highest caster level is 10 (5 from wizard + 5 diabolist) + your 5 levels of diabolist added on. that doesn't seem correct RAI, but that's how it reads to me. maybe it should be 'highest caster level apart from diabolist levels'
Actually, that is how I read it, but I highly doubt that was the intent, so I'm just going to run with the other unless I hear otherwise.