Note: Per Patch 2.1.9 - Misc Fixes, Spell Tolerance is currently disabled.
Spell Tolerance is a pretty unique concept we created to tackle two issues:
a) It can be incredibly difficult for players (especially dexers) to go toe-to-toe with several high-level casting creatures simultaneously, and usually results in players always opting to instead play as bards and tamers simply because spell damage is so overwhelming
b) Large group fights typically devolve into non-stop offscreen kiting and near-instant sync dump kills
So What is Spell Tolerance?
Spell Tolerance is essentially a list of the spells that have been casted on a player over the last few seconds, namely:
1) Who casted the spell (was it a Player or a Creature?)
2) What specific spell was it (Energy Bolt? Fireball? Mind Blast?)
3) How long ago was the spell casted (or if it has a delayed effect, like explosion, when it was resolved?)
What Spell Tolerance effectively does is reduce the damage that a player receives from spells, based on what has previously been casted on them. However, spells casted onto a player from other players and spells casted onto a player from a monster are resolved somewhat differently.
Players Casting Spells on Other Players
One of the goals of the Spell Tolerance system is to NOT effect small-scale combat. We don't want duels to be affected by the system or to have small fights significantly impacted by this system.
As such, spells cast by a player onto another player exist in the Spell Tolerance list for only 1 second long, and are then removed from the list afterwards. Additionally, the first four spells that appear in a player's Spell Tolerance list that were player-casted are also considered "ignored" by the system when calculating damage reductions (explained later).
Example: A player precasts Explosion, targets it, and then casts ebolt onto another player. The two spells fire off simultaneously on the target and are now added to the target's Spell Tolerance list (which now has 2 spells in it: Explosion and Ebolt). The player then casts lightning and targets that same player with it. However, the lightning spell takes longer than 1 second to cast and target, and by the time it fires off, the Explosion and the Energy Bolt have already been removed from the targeted player's Spell Tolerance list (as player-casted spells only last for 1 second in the list).
So in a 1vs1 fight, it's practically impossible to get Spell Tolerance to fire off, with the exception of possibly reducing the damage of Magic Arrow and Harm (both of which do no damage anyways, so it's a moot point).
So when a spell hits a player, we look at all the other player-casted spells currently in that player's Spell Tolerance list (which are still only spells that have fired off within less than a second ago) and we ignore the first four spells found in that list.
After ignoring those first four spells in the list, damage is reduced as such:
-5% Spell Damage reduction for each spell in the list -5% additional Spell Damage reduction for each time a specific spell type (such as Ebolt, Fireball, etc) appears in the list again after the first instance
Let's say the following spells are all cast, in this order, and resolve onto a single player all within a 1 second window:
Explosion: 100% Damage (1st spell: so it's ignored) Explosion: 100% Damage (2nd spell: so it's ignored) Explosion: 100% Damage (3rd spell: so it's ignored) Ebolt: 100% Damage (4th spell: so it's ignored) Ebolt: 90% Damage (5th spell, 2nd time Ebolt appears in the list) Ebolt: 80% Damage (6th spell, 3rdd time Ebolt appears in the list) Lightning: 85% Damage (7th spell) Lightning: 75% Damage (8th spell, 2nd Lightning in list)
As you can see, as you scale up in total casters, there definitely is a significant reduction in damage, and while those damage numbers for 5-7 casters are still very high, with ebolt distance-based damage delays, you may be able to get some greater heals into the mix now and prevent those characters from dying (at some for some scenarios that previously were instant death).
Creatures Casting Spells on Players
The system works in the same way for spell reduction when creatures casting spells onto players, however with a few notable differences.
1) Spells casted by creatures and players are treated totally independently in the list (players will not receive PvP spell damage reductions from spells in the Spell Tolerance List that came from creatures and vice versa)
2) Spells casted by creatures last in the Spell Tolerance list for 10 seconds, instead of 1 second
3) When looking at creature-casted spells in the Spell Tolerance list, only the First spell in the list is ignored, not the first four.
This allow players to build up a bit of a damage reduction over a few seconds when being peppered by multiple spell casting creatures simultaneously.
Also of note: The maximum total spell damage reduction that can be reached from both creatures and players is -75% spell damage. However in PvP it is astronomically unlikely you'll get anywhere near that damage reduction (as shown in that chart above), but it's possible in PvM if a player was swarmed by like 20 imps.