Twisted Fate
Twisted Fate is a temporary trait, sort of like a status effect. A unit who has the trait will be completely spared from the next instance of damage to hit them; after the DRNs for hitting and damaging are counted, the damage is set to zero.
A hit that does zero or one damage will not trigger Twisted Fate, nor will a hit that does more than 900 damage. The only hits that do 900+ damage are "instakills" such as Soul Slay, which do 999. While there are currently no "instakills" that interact with any physical or elemental resistances, if there were, the programming dictates an "instakill" that the target has at least 10% resistance to has a decent chance of being stopped by Twisted Fate. (For example, a fire "instakill" would likely be blocked by Twisted Fate if the target has at least 5 
Fire Resistance.)
Unlike Luck, Twisted Fate can be applied to anyone; however, applying it to Inanimates or the Undead is relatively tricky in the late game, requiring a more-specific application.
Twisted Fate is an Astral buff provided by the Twist Fate spell, the Gift of Cheated Fate spell (Alteration 2), the Will of the Fates spell (Alteration 7), or the Rabbit Foot Charm. The first two spells and the item require 
1, while Will of the Fates requires 5 and 2
(or 4 and 3). The item is of particular note, since it protects against remote attack rituals such as Seeking Arrow (though only the first of such attacks each turn).
Fool's Luck is pretty similar to Twisted Fate, in that it kicks in against all damage of two points or greater (or even one point of damage if the subject has under 3 HP); however, it's a permanent trait, and every second "block" doubles damage instead.