How do you get an Imp familiar for a cleric?

[NotMousse]

I'd seen someone with a build for this, but after skimming the cleric entry didn't see anything that allowed for this. Did I miss something?

[Ringtail]

You can get an Imp Companion that scales in level like a Druid's Animal Companion by going Diabolist* from Princes of Darkness.

Was that what you were looking for?

*Prestige class for Clerics and/or Sorcerers/Wizards

[NotMousse]

Lemme guess, the Cheliax book?

[Ringtail]

Not quite. Book of the Damned Vol. 1, unless I'm mistaken.