Once you hit artifact level 69, 72, and 75, Tier 3 of the Netherlight Crucible will unlock for the first, second, and third relic of your artifact weapon. You'll be able to add one rank to a minor trait per relic.
Three traits will be presented, but the Tier 2 power you choose (left or right) determines the path to the Tier 3 traits, meaning you'll only be able to pick between two of the Tier 3 traits (Left-Middle/Middle-Right). This means it may be worthwhile to pick the weaker Tier 2 power in order to get one of your best traits.
Just like relics, you will only be able to choose a trait that has multiple ranks (a 4/4 trait). You will also never see the trait that is on the relic you're looking at in the Crucible (e.g. if your relic has Toughness, you'll never see Toughness as an option in the Crucible for that relic). There are 10 minor traits, meaning there are 9 possible traits to choose from (3 of which you'll see). If you only have one "best" trait, there is a 33.3% chance you'll see it. If you have two, there's a 58.3% chance. Three, 76.2% chance (1 - (6/9) * (5/8) * (4/7)). Your odds of seeing something you want aren't too bad.
To make comparisons simpler and easier to visualize, I'm looking at what each trait gives you when going from 4/4 to 5/5.
http://www.wowhead.com/spell=238077/bastion-of-the-aspects - Increases the damage prevented by blocking by 2% per rank.
This is an additive bonus, meaning if you have 4/4 of this trait, your blocks are now 38% additional DR instead of 30%, and your crit blocks are 76% additional DR instead of 60%.
Let's say we're taking attacks for 2 million damage after all DR. Gaining another rank in Bastion of the Aspects gives us an additional 2% DR for blocking. If we block, 40,000 damage is prevented. If we crit block, 80,000 damage is prevented. While this entirely depends on stats and how much Mastery you have, we can reasonably expect that 50% of blocks are crit blocks. Per hit, an additional rank to Bastion prevents, on average, 60,000 damage in our example. Also, since this is percent DR, it increases in strength as the amount of damage increases.
http://www.wowhead.com/spell=188635/vrykul-shield-training - Increases armor by 2% per rank.
This is a bit trickier to figure out as an additional 2% armor does not mean 2% DR. Continuing with our 2 mil per hit example from above, our hypothetical Pror Warrior has 10,000 armor. An additional 2% armor gets us 10,200 armor.
The formula for armor's damage reduction is as follows: armor / (armor + attackerConstant)
The attackerConstant being 8164 for a level 113 enemy (boss).
(10200 / (10200 + 8164)) - (10000 / (10000 + 8164)) = ~0.49%
An additional 200 armor gives us an addition 0.49% DR. 2,000,000 - 2,000,000 * 0.9951 = 9800 damage prevented per hit in our example.
http://www.wowhead.com/spell=203225/dragon-skin - Increases the total damage ignored by Ignore Pain by 2% per rank.
The formula for a max rage (effective absorb) Ignore Pain is: AP * 20.07 * (1 + Dragon Skin increase) * any other bonuses (that don't matter for this comparison).
We have 55,000 AP. (55,000 * 20.07 * 1.1) - (55,000 * 20.07 * 1.08) = 22,077 extra absorb per Ignore Pain.
http://www.wowhead.com/spell=188632/toughness - Increases Stamina by 1% per rank.
If we have 115,000 stamina with 4/4, that gives us 116,150 stamina with 5/5 Toughness. That's a health increase of: (116,150 * 60) - (115,000 * 60) = 69,000.
http://www.wowhead.com/spell=188683/will-to-survive - Last Stand increases health by an additional 10% per rank.
If we have 7,000,000 health, 4/4 Will to Survive gets us to 11,900,000 health (7,000,000 * 1.7). 5/5 gets us to 12,600,000 health. That's a 700,000 health increase. The "issue" with Last Stand is that it's generally used to survive what would otherwise one-shot you, or as an "oh shit" button. It doesn't provide a consistent survivability increase like the other traits, and as such doesn't really compare.
Comparing the survivability traits
Comparing Dragon Skin to Bastion of the Aspects isn't something you can do at a glance. To compare them we can look at the total effect over a given time period. Let's say we have a fight that lasts 6 minutes. Going off my Legendary Comparison sheet, Ignore Pain will be cast ~120 times. If we're taking melee hits for half the fight and the boss attacks every 2 seconds, that's 90 melee hits taken. It's not unreasonable to assume that every hit will be blocked.
Bastion of the Aspects: 90 * 60,000 = 5,400,000 damage prevented
Vrykul Shield Training: 90 * 9800 = 882,000 damage prevented
Dragon Skin: 120 * 22,077 = 2,649,240 extra absorb
Toughness is a bit interesting in that it's a health increase. If we compare it to, say, Dragon Skin for a single hit, it's clearly more EHP. However, in my opinion, the total absorb you get over the course of a fight outweighs the EHP you get from Toughness.
A couple notes:
- This obviously is ignoring several variables (like damage taken besides melees) that could alter the results of these calculations, but the difference between the traits is so (relatively) extreme that the rankings won't change.
- The above scenario assumes you're taking Vengeance. Say you take BV, that will increase the value of Vrykul Shield Training in comparison to Dragon Skin as you're not going to cast Ignore Pain as much. Vrykul Shield Training and Dragon Skin are both good, but in most cases Dragon Skin will provide more survivability.
http://www.wowhead.com/spell=216272/rage-of-the-fallen - Increases damage dealt by Revenge by 6% per rank.
Revenge's damage is 350% of AP.
(55,000 * 3.5 * 1.30) - (55,000 * 3.5 * 1.24) = 11,550 damage
Revenge is cast ~108 times.
108 * 11,550 = 1,247,400 extra damage
http://www.wowhead.com/spell=188644/thunder-crash - Increases damage dealt by Thunder Clap by 10% per rank.
Thunder Clap's damage is 300% of AP.
(55,000 * 3 * 1.50) - (55,000 * 3 * 1.40) = 16,550 damage
Thunder Clap is cast ~50 times.
50 * 16,550 = 827,500 extra damage
http://www.wowhead.com/spell=188639/shatter-the-bones - Increases the critical strike chance of Shield Slam by 5% during Shield Block per rank.
Let's say we have a Shield Block uptime of 75%. Shield Slam is cast ~100 times. 75% of those casts are inside of Shield Block. If we have 10% Crit, Shield Slam has a 30% chance to crit with 4/4. So 30% of our 75 Shield Slam casts will crit.
Shield Slam's damage is 492.8% of AP.
4/4: 0.3 * 75 = ~22 Shield Slam crits
5/5: 0.35 * 75 = ~26 Shield Slam crits
We also have to account for the fact that Shield Slam's damage is increased during Shield Block. 25 casts are outside of Shield Block.
25 * 55,000 * 4.928 = 6,776,000
75 - 22 = 53 non-crit casts inside Shield Block. 53 * 55,000 * 4.928 * 1.3 = 18,674,656
22 crit casts inside Shield Block. 22 * 55,000 * 4.928 * 1.3 * 2 = 15,503,488
6,776,000 + 18,674,656 + 15,503,488 = 40,954,144 total damage
25 * 55,000 * 4.928 = 6,776,000
75 - 26 = 49 non-crit casts inside Shield Block. 49 * 55,000 * 4.928 * 1.3 = 17,265,248
26 crit casts inside Shield Block. 26 * 55,000 * 4.928 * 1.3 * 2 = 18,322,304
6,776,000 + 17,265,248 + 18,322,304 = 42,363,552 total damage
42,363,552 - 40,954,144 = 1,409,408 extra damage
Comparing the damage traits
Despite Thunder Crash providing more damage when comparing a single cast of each, you're going to be casting Revenge far more frequently, adding up to more total damage. If you don't have Vengeance, the gap between Thunder Clap and Revenge casts will close, but in general the damage gained from one more rank of Rage of the Fallen will be more than Thunder Crash.
In our example above, Shatter the Bones is better for single-target damage compared to the other two traits, however, there are a couple factors I did not account for. e.g. AM vs. HR (which changes SBlock uptime) and Battle Cry (which reduces the value of StB). With those things taken into consideration, StB is still likely better for single-target, but still very close to Rage of the Fallen. With Rage of the Fallen essentially doubling in value for every target, there's no reason to choose StB over it. The same can be said for StB vs. Thunder Crash.