Will Thrasher's Practical Magic spell, Golden Silence, got me thinking of the Runequest applications of Vows and silence. I already wrote up a sorcery spell, Silence/ Stifle. But an Oath sounded more like a spell for cultists to take in the presence of their god. So here is Oath of Silence - a Rune spell for those devoted to a god with the truth Rune  [Humakt and Lankhor Mhy, but also others].

OATH OF SILENCE [Truth Rune Spell] 

Rank: Initiate, Rune level

Duration until discharged, then

Range Self or Person Led through the Ritual

Target one creature touched that is capable of speech

One person, which may be yourself or another Initiate or Rune level of your cult, must pray at a temple to your god, swearing a temporary Oath of Silence. The person taking the Oath, must dedicate Temporary POW [Magic Points] to the spell. These POW charge the Oath. The longer the target goes without breaking this vow, the greater the spell's effect when discharged. This spell is discharged by any act that violates the vow of silence (e.g. no speaking, spell-casting, singing, whistling, or making any other sounds with the mouth).

When the spell is discharged, whether intentionally or not, for every full day the Oath was kept, the Oathsworn receives the benefits of 1 point of each of the following per day the Oath was maintained, to a maximum equal to the level of the Oath of Silence rune spell used in the initial ritual:

• Bladesharp or Bludgeon [Oathsworn’s choice]
• Countermagic Shield
• Protection

Thus, if the Oathsworn maintained the oath for 3 full days, then spoke when attacked, he would immediately gain the benefit of having cast Countermagic Shield 3, Protection 3, and his choice of Bladesharp or Bludgeon 3, without having to spend time casting these spells. 

The Oathsworn need not know these spells in order to benefit, nor may she cast them again if she does not. Once the spells are in place they behave as normal in terms of duration, etc.

Note: If the target is tempted or coerced to break their vow (e.g. torture, an insult to their honor, enduring great pain, etc.), they may make attempt POW x3 to maintain their Oath. If the target is physically incapable of speech the Oath is dispelled immediately.