One way you could do it that I find intriguing:
- Give two options for base creatures one for good warlocks and one for evil ones.
- Apply the Maurid template to it, homebrewed to make the Warlock the id-core.
- The creature is an echo of the warlocks self. It wears the same equipment as the warlock but all effects are only half as strong (+2 chainmail, becomes a +1 chainmail with a base 2 armor bonus).
- The creature advances in hitdie based on the warlocks caster-level.
- Every time it reforms it gains access to one invocation of the each level of invocation the warlock can cast except the highest. It can use these invocation 1/minute each.
- At level 12 it gains the fiendish/celestial creature template free of charge.
- At level 17 the warlocks power has reached a level that allows his echo to reach greater power converting it into a Vrock/???
If taken, this invocation always occupies one slot of the highest level of invocation the warlock can use.
The reason I am powering up the invocation is because the 24hour downtime for reformation can very much cripple its utility function.
Yes, this makes it more of a power invocation(though I haven't done calculations how the template works in tandem with innate abilities).
Originally I just wanted to suggest giving the quasit a template at level 12 but then I kept writing.