Devil's Advocate

Burning a crow's feather in ignited oil in the shape of a Patron's symbol allows the Warlock a quick parlay with an Underling of their Patron. The Underling does not take physical form, but rather appears as they would in the Prime Material Plane, their shape apparating from the swirling, dark grey smoke.

The spell ends when the oil ceases to burn, (roughly 5 minutes). Any additional time for parlay requires another vile of oil and another crow's feather. 

As the Underling is only ephemeral in the conversation, and does not take physical form, the Underling cannot be harmed, or harm the caster, and cannot be affected by spells such as Zone of Truth or Suggestion, nor can they affect the caster in such a way.