As the title hints at, I'm trying to get this build to abuse the hell out of Mark of Cania to produce ridiculous checks. I originally had a Humatula irritating a vestage and poking a horse for a day, but that didn't produce the result wanted with absolute certainty. So I have moved on to an Imp trying to resist its own power.
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Race: Imp
Found in the sample monster classes in Savage Species. It's already a devil, so it will not have any problems taking Brand of Nine Hells as a feat. Just pledge your allegiance to Mephistopheles, wait a while, take Mark of Cania at ECL 4.
Just use the standard array and throw the 11s into your physical stats and your 10s in mental stats.
ECL 4 is also when the Imp gets the ability to cast Invisibility at will, so long as it only targets itself. Now, Mark of Cania triggers whenever a target of your spell-like abilities makes a successful save (among other situations); Invisibility is a spell-like ability, you are a target. Even though it is harmless, you can attempt a save.
Let's begin. Use your invisibility ability, attempt will saves. Succeed, say 12 times in a row. It may take a little longer than twelve castings of your ability due to a chance of failure, but it will happen eventually. Have +12 to all skill checks.
Next, find a wizard offering spells, purchase a casting of a CL 20 Extended Heroics (600gp) and nab Martial Study (Moment of Perfect Mind). Spend the next 400 minutes succeeding in concentrating against your invisibility (you'll succeed every time with your always at least 13 concentration check as a will save against the DC 12 save (10+2{emulating second level spell}+0{CHA}))
Now that the spell has worn off, you have accumulated a +412 on all attack rolls, damage rolls, saves and checks along with a very low AC.
Lastly, demand free services from the wizard (you have a +412 on your diplomacy). Repeat castings of Heroics for absurdly high checks and saves, aim for about a +A(g64, g64improve on this little devil?