I finally got on the PTS with my existing characters to do some testing and discovered something very troubleing.
*Note: Details on my testing method can be found at the bottom of the post.
Class abilities that now scale off of stamina are no longer considered "spells". And, of course they are not "weapon skills" either.
These abilities are now unclassified and stuck in a no-man's land of incompatibility with 5 piece armor sets and other game mechanics.
** Armor bonuses that "add X to spell attacks" or proc "X after a spell attack" will NOT WORK with class abilities based on stam.
This is true for offensive AND DEFENSIVE buffs. (Offensive such as Stygian, Defensive such as Syrabane's Grip or Buffer of the Swift)
** This may explain why some people are getting hit so hard in PvP. Your "spell resist" is not lowering the damage of spells that scale off stamina.
I believe it's not just the armor set bonuses. Logically, I believe they must also be incompatible with the overall game mechanics that drive these sets. This is harder to test, because I need a partner from an opposing alliance to attack me multiple times with different variables.
ZOS: Please fix this. Even if class abilities now scale off stamina, they should still be considered "spells".
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Testing Method:
Pretty straight forward. I was wearing a full 5 piece set of Stygian armor that when invisible causes "spells to do an additional 20% damage". I crafted an identical unnamed armor piece of the same weight, trait, enchantment and quality.
First I used the magica based morph of a class ability to strike an NPC out of stealth wearing 4 pieces of Stygian and 1 piece crafted for 4018 damage. I repeated the process with 5 pieces of Stygian for 4821 damage.
Next I respec'd and used the stamina based morph of the same class ability to strike the same NPC with the same conditions using the same set. This time there was no change in damage.
I testing this several times, repeated on different NPCs, and always got the same result.