Is there a longer lasting Geas or "oath" ability/spell/item out there?

[Arravis]

Is there an ability or spell that tries to enforce an "oath" a creature has voluntarily taken? For example, if a king assigns a group of wizards to investigate treason within his own empire, but the king doesn't want them to report their findings to anyone except the king himself. Anyway, is there anyway (in any spell, ability, item, etc) to somewhat "guarantee" their oath? Geas only lasts for 1 day per caster level... not long term enough. Any suggestions? Thanks guys .

 

[Joshua Randall]

A cleverly worded mark of justice (Clr5, Pal4) might do the trick. Duration: permanent.

There's always miracle or wish if you are really desperate. I think a long-duration geas effect would be reasonable for a wish (maybe even for limited wish).

 

[DaveStebbins]

If the geas is undertaken willingly, of the character's own free will, you can house-rule a longer duration. I think it's reasonable to assume that the PHB duration is for unwilling targets, and house rule a longer one for others.