TL;DR:

Basic tests:

As expected, Minor Slayer produced a 5% damage increase to both my basic weapon attacks and my abilities in 4-player group dungeons, 12-player trials, and the two arenas.

It works in both the scored vet and unscored normal versions of the trials/arenas.

It works against every tested enemy in the dungeons/trials, even human ones (in case there was any confusion about what constituted a "Monster")

It does not work against overworld enemies.

It does not work against the World Boss.

It does not work in delves.

It does not work in public dungeons. (so the tooltip's use of the word "Dungeon" is a bit ambiguous here)

Minor Slayer is currently broken on console. (Thread #1, Thread #2)

Minor Slayer stacks additively with execute damage. See @Asayre's post #41 in this thread.

Old vs. New:

No armor, vMA staff, and a 3p jewelry set with spell damage enchants: purple arcane Willpower, gold arcane Infallible, and gold arcane Moondancer

5p Julianos, 2p Nerien'eth, vMA staff, and a 3p jewelry set with spell damage enchants: purple arcane Willpower, gold arcane Infallible, and gold arcane Moondancer

Crit:

Update:

Conclusion:

Update 11 saw the Minor Slayer buff changed from affecting "Boss Monsters" to "Dungeon and Trial Monsters".Yes, Minor Slayer works (mostly) as advertised.I ran a series of basic tests. I would attack enemies while wearing 3p Moondancer jewelry, and then attack them again after having swapped one of the Moondancer jewelry pieces with an Infalliable Aether jewelry piece. Replacing one jewelry piece with an identically-statted jewelry piece of a different set causes me to lose just the Minor Slayer buff while keeping everything else the same. I would note and compare the exact damage numbers with and without Minor Slayer.I tested this against enemies in Spindleclutch, Sanctum, DSA, Maelstrom, gargoyles in Craglorn, and finally, a World Boss, Public Dungeon, and delve in Eastmarch.The most interesting question is whether a 3p Minor Slayer jewelry set will outperform a 3p Willpower/Agility set. For simplicity, I'm not considering the full 5p set in this post--just a 3p jewelry set that can act as a drop-in replacement for 3p Willpower/Agility.The Willpower and Agility sets are quite powerful because their set bonuses are 50% stronger than the typical set bonus. However, they provide a flat bonus to your base damage, whereas Minor Slayer provides a percentage bonus. So at a certain point, your base damage stats will be high enough that Minor Slayer will outperform Willpower/Agility.To get a rough idea of what this looks like, I did a few tests.The results are here:First, the tests without any armor were done to confirm the expectation that, with lower base stats (missing set bonuses and missing gear enchantments when going armorless), the results skew more towards Willpower's favor (and it did, slightly). And this was also a way for me to see just how much base stats could skew the Slayer vs. Willpower/Agility comparison (not by that much) and thus how applicable this is to other builds that have different base stats (quite applicable).Second, with my typical dungeon setup, 3p Minor Slayer (via Moondancer) was able to outperform 3p Willpower by 1.3% for abilities. Staff attacks disproportionally favor spell damage, so Willpower wins has a slight win there. On the whole, 3p Minor Slayer (via Moondancer) represents approximately a 1% DPS increase over Willpower.So far, I've only compared Moondancer to Willpower, because the crit chance remains the same, so all you need to do is compare how much each attack hits for.Whereas Moondancer has max magicka as its 2p bonus, Infallible has a 3.1% crit chance as its 2p bonus. The expected DPS from extra crit is calculated by (crit chance * crit damage bonus), and for my magicka nightblade, I have a base crit damage bonus of 0.74. So the 3.1% crit bonus from Infallible translates into 2.29% extra DPS. Different classes and builds have different crit damage bonuses (Nightblades and Templars have passives that increase this, and the number of points you put into Elfborn or Precise Strikes will affect this as well), and I have not taken into account the Minor/Major Force operational bonuses.So while 3p Infallible has 1.88% less staff damage and 0.96% less ability damage on my nightblade than 3p Willpower, its crit chance is worth 2.29% extra damage on my nightblade, and on the whole, it is a DPS increase over Willpower. This difference in favor of Infallible would be even higher with Minor and/or Major Force active, such as when a War Horn is active, and when there are additional bonuses of crit damage (e.g., running Shadow with Twice-Born Star).Versus Moondancer, the difference is quite minimal on my nightblade's build: the 2.29% extra damage from crit chance cancels out the 2.27% baseline advantage that Moondancer has over Infallible. Things swing more in Infallible favor when considering staff attacks, when adding in Minor/Major Force, and if I were to use TBS instead of Julianos.Rather unintuitively, the Minor Slayer buff stacks additively to the damage increase from execute. More details can be found in @Asayre's post on the second page . This was not a scenario that I had considered in my original tests, so a big thanks to @Asayre for uncovering this problem.In short, it means that Minor Slayer's 5% is calculated from the base pre-execute damage and does not buff any of the execution bonus. This is rather unintuitive, and it's unclear if it is intended or an oversight by ZOS. Whatever case, the way in which Minor Slayer works right now, its performance is very lackluster for builds that deal a lot of execute damage (magicka Templars in particular).While these numbers and tests are specific to my magicka nightblade's build, I don't expect them to be that different for other builds and classes.For stamina users, 3p Alkosh or 3p Vicious Ophidian with Minor Slayer should slightly outperform 3p Agility. (These two sets have the same 2p and 3p bonuses.) (Though for stamina, people should seriously consider running the full 5p Vicious Ophidian.)For magicka users, both 3p Moondancer and 3p Infallible should both slightly outperform 3p Willpower, with Moondancer doing more damage per hit than Infallible, and Infallible getting more crits. Depending on how much crit damage you have, Infallible will either perform on par with Moondancer or somewhat better (particularly in trials with TBS with War Horn).Of course, this doesn't necessarily spell the doom of Willpower/Agility: you will still want to use those sets for PvP, and in PvE, the advantage of the Minor Slayer sets apply only for the 4-person group dungeons, the 12-person trials, DSA, and MA (though those are the only pieces of PvE content worth optimizing for).