Here's an idea. Don't know if it's 8th-level material, but it's pretty powerful without necessarily being a "plot-wrecker" for DM's to worry about. There might be some game-breaking logic loopholes I haven't thought of.
Truth Conversion
Divination
Level: Sor/Wiz 8, Knowledge 8
Components: M
Casting Time: 1 action
Range: Personal
Target: You
Duration: 1 minute/level
Saving Throw: None
Spell Resistance: No
While under the effects of truth conversion, the caster not only knows all lies she hears for what they are, but hears in her head the truth that the liar is concealing. For instance, if someone knows that a prisoner is concealed in the top of a tower, but claims that the prisoner is being kept in a basement dungeon, the caster of truth conversion will hear in her head: "The prisoner is in the top of a tower." (Note that the caster physically hears the lie, but simultaneously gains an inner knowledge of the truth.)
If the speaker of a lie honestly believes he is telling the truth, the caster of truth conversion will know that what is spoken is a lie, but not know the truth about the subject. A liar who does not know the truth about what he speaks will produce a similar result. (This prevents the caster from learning secrets by speaking random lies about some subject.)
Note that the truth heard by the caster is generally equal and opposite to the lie spoken. So, in the above example, if the liar had said: "the prisoner is dead," the caster would learn only "the prisoner is alive," not "the prisoner is alive and in a tower." The more (false) detail the liar provides, the more truth the Diviner learns.
The caster need not understand the language being spoken to learn the truth of what is said to her, although she doesn't get any special understanding of the lie itself if the language is unfamiliar to her.
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Why this spell is 8th level:
- note the lack of verbal or somatic components. As long as the matetial components are on the person of the caster, this spell can be cast undetectably.
- because the spell is targeted on the caster and not the potential liar, there is no saving throw, and it bypasses anti-divination measures cast personally upon the liar. Likewise it would work on lies uttered by constructs, undead and elementals.