As an optimization exercise, I'm trying to get as much sneak attack as possible shoved into a 20-level build. Here's what I've got...
Loredrake DWK Trickster Spellthief 6/Sorcerer 1/Heartfire Fanner 1/Divine Oracle 1/Unseen Seer 1/Sublime Chord 2/Unseen Seer +3/Archmage 5
Feats: Master Spellthief, Practiced Spellcaster (Sorcerer), Dragonwrought, Greater Rite, Planar Touchstone (catalogues of enlightenment), Magic Sensitive, whatever feats for prereqs.
Base sneak attack of +1d6 (spellthief) +2d6 (unseen seer). Then, we cast fiendform, followed by polymorph into a Kelvezu for +8d6 = +11d6.
Finally, we cast Hunter's Eye, grabbed with Unseen Seer. Note that with MS we can stack spellthief levels with sorc CL and sublime chord CL for arcane spells. CL calculation:
Sorc CL = 4 base +2 loredrake +1 rite +4 practiced = 11
SC CL = 10 base +11 sorc = 21
Total CL = 6 spellthief + 11 sorc + 21 SC + 1 unseen seer +2 oracle domain +1 reserve feat +5 archmage +1 knowledge domain +5 suffer the flesh (why not) +1 ring of arcane might +1 robe of arcane might (divination) +1 orange ioun stone +1 vial of the last gasp = 57
So we're at +30d6. What else?