A Formal Promise is made by a faerie who intends to keep it.
The Oath Promise Ceremony Edit
“By root and branch, by leaf and vine, on rowan and oak and ash and thorn I swear that what is given to my keeping shall remain in my keeping and shall be given over only to the one who holds my bond. My blood to the defense of the task I am set, my heart to the keeping of the promise to which I am bound.” The air grew thick with the taste of pennyroyal and musk as his magic crackled around us, drowning out the taste of roses.
“Broken promises are the road to our damnation,” I said, the copper and cut grass smell of my own magic undercutting his. “Promises kept are the meeting of all our myriad roads.”
“And such a meeting will my promise be.” 
"I'll swear. I swear by oak and ash and rowan and thorn that we did not come here intending harm. I swear by root and branch and rose and tree that none will raise a hand against you, unless hands are raised against us."