During combat, Resistance to All increases the Resistance score of all friendly units on the battlefield by a set amount. This amount is not reported in-game, but this wiki adds the size of the bonus after the name of the ability to indicate this, rendered as "Resistance to All +1", "Resistance to All +2", et cetera.
Bonuses from Resistance to All do not stack. Therefore, if two units possess this ability, only one (with the highest-strength ability) applies its bonus to all friendly units. It similarly does not stack with the Hero Ability "Prayermaster".
There are only 2 units in the game that possess this ability: the Guardian Spirit and the Unicorns. Both are Fantastic Units belonging to the Life realm. No unit may acquire Resistance to All if it does not have it by default, though the Hero Ability of "Prayermaster" performs largely the same function and can be acquired by certain Heroes who do not possess it by default.
A unit's ability to evade ill effects is measured through its "Resistance" score. Many negative effects in the game may have a certain chance to affect (or fail to affect) a creature depending on that creature's willpower - its burning desire to stay alive and keep fighting. Some creatures actually inspire others, improving their will to survive, and in so doing increase their chance of averting such effects.
When such a creature is present on the battlefield, its very presence is inspiring. It does not have to do anything to win its friends' trust - mainly in themselves.
The bonus equals the strength of the Resistance to All ability. For example, if a unit has Resistance to All +1, each and every friendly unit on the battlefield (including itself) receives a +1 Resistance bonus.
As a result, each and every friendly unit becomes harder to affect with Curses, as well as some types of spells and Special Attacks which require units to perform Resistance checks. Though bonuses from Resistance to All are usually very small, they can be very significant - and the greatest benefit is that they apply to all friendly units.
If the Resistance to All unit dies or is removed from combat, its bonus to all friendlies is immediately lost. Note also that this bonus applies only during combat: the unit does not raise its own Resistance score nor the Resistance of any unit in its army stack while traveling on the overland map.
Resistance to All bonuses do not stack. If there are several units with Resistance to All fighting together in the same army, the game will pick the one with the strongest Resistance to All ability and apply bonuses only from that ability. For example, if one unit on the battlefield has Resistance to All +1, and another has Resistance to All +2, then only the +2 bonus is applied.
The check to see which unit has the best Resistance to All is done constantly, so if any such unit dies, the bonus is immediately recalculated. Taking the previous example, if the unit with Resistance to All +2 dies, the unit with Resistance to All +1 is now the strongest on the battlefield, so all friendly units now get +1 instead of +2. If that unit dies as well, friendly units no longer receive any bonuses.
Resistance to All and Prayermaster Edit
Prayermaster and Resistance to All do not stack. The same algorithms are used for determining which bonus to add: selecting the strongest bonus available on the battlefield at the time, and ignoring all others. For example, if one unit on the battlefield has Prayermaster +3, and another has Resistance to All +2, then only the +3 bonus is applied.
Units with Resistance to All Edit
There are exactly two units in the game that possess the Resistance to All ability. Both are Fantastic Creatures from the Life realm:
A Guardian Spirit has Resistance to All +1, while Unicorns have Resistance to All +2. There are no other units with Resistance to All, and there is no unit with a stronger bonus than Resistance to All +2.
Note however that some Heroes possess Prayermaster, which is largely identical and (as explained earlier) does not stack with Resistance to All either. The main advantage of Prayermaster is that its strength grows with the Hero's Experience level, and can potentially become much more powerful than any available Resistance to All ability.
Acquiring Resistance to All Edit
There is no known method by which to acquire Resistance to All for a unit that does not possess it by default. There is also no way to increase the strength of a unit's Resistance to All ability.