Muffle

 is an Apprentice-level Illusion spell in The Elder Scrolls V: Skyrim. With a base casting cost of 127 Magicka, casting the spell renders the caster's footsteps completely silent. The spell lasts for 180 seconds when single-casted or 450 seconds when dual-casted.

Invisibility
Because the Dragonborn's footfall can be heard by enemies while invisible, Muffle is useful when combined with it. Using both spells makes the Dragonborn almost impossible to detect by others, unless the Dragonborn bumps into them or performs an action. The player may also be detected when entering a scripted scenario like meeting with a courier or any other character that engages in dialogue with the Dragonborn.

Spell Tome locations

 * Farengar Secret-Fire sells this spell at Dragonsreach in Whiterun.
 * Drevis Neloren, of The College of Winterhold sells this spell when the Dragonborn reaches level 25 Illusion.
 * Calcelmo sells this spell at Understone Keep in Markarth.

Enchanting
This effect also appears as an enchant capable of being obtained by disenchanting apparel with the effect. This effect is only available with randomly generated vendor items and found items and does not include unique items such as Ebony Mail or Nightingale Boots. It appears that the passive enchant bears half the effectiveness of the spell version (25% versus 50%, presumably corresponding to the 50% figure that is reduced by the Sneak perk, Muffled Movement.

Trivia

 * This magic spell seems to increase the Illusion skill abnormally fast, which may be useful to power level.