féth fíada

féth fíada, feth fiadha, fé fíada, faeth fiadha [OIr. féth, mist, fog; fíada, lord, master, possessor]. A magic mist or veil that usually renders those under it invisible; sometimes those under it may take animal form. Also known as ceó druídecta, ceo draoidheachte [druid's fog]. Féth fíada is usually thought to be a power of druids and the Tuatha Dé Danann, given them by Manannán mac Lir after their defeat by the Milesians. Eithne (3) had féth fíada but lost it when she partook of forbidden food; see ALTROM TIGE DÁ MEDAR [The Nurture of the Houses of the Two Milk Vessels]. The Fenian hero Caílte was wrapped in féth fíada when seeking a physician from Angus Óg.

The power was thought to have passed to Christian saints, an attribution persisting in Scottish Gaelic oral tradition until recent times. In the celebrated prayer-poem ‘The Deer's Cry’ or ‘St Patrick's Breastplate’, attributed to St Patrick, the saint turns himself and his companion Benén into wild deer on their way to evangelize Tara. The enemies who wished to ambush them saw only a deer with a fawn.