Going to throw up a theory... please shoot it down

Anyway I am thinking that the Spriggans all have
DARKNESS. Note: When Crunchyroll translations come out I will add to/edit this post.
There's been a question about the magic power of the 12 - how/why are they so strong?
The 12's relative MP level:
- It seems that Isghal's best wizards were a fair way below the 12. Hyberion, Wolfheim, Warrod and Jura were outlier wizards from Isghal, and together they were still outclassed by 1 Spriggan level mage. This mage was from Isghal (Serena), so Isghal produces one but Alvarez 11?
- Within Alvarez there seems to be a huge gap between a shield and their subordinates. Marin is probably the best of the subordinates, and he isn't exactly a stud. Why is there such a gap? Isghal doesn't have a gap that is so glaringly large. For example compare Laxus to Erza, or Hyberion to Jura..
What can possibly explain the immense magic power of the 12?
Invel said this in the most recent chapter...
- Invel calls darkness a fearsome power
- Darkness (a quality of the heart) is a seed to black magic
- The line where Invel references the power to the Emperor is a little weird (I will await the C'roll translation)... but Gray's response to that is "you know you're bad guys". Gray thinks darkness = bad guys, which Invel corrected. But from Gray's reaction he said 'you' as in the 12. This implies that the 12 are also dyed in darkness... which is what Invel is hoping will happen to Gray after he kills Juvia.
In short..
I believe that the Spriggan's are so powerful because they are
black wizards, or they have been taught how to harness the
power of darkness. Zeref is the leader of Alvarez. He's the legendary Black wizard. And he assembled the 12 to combat Acnologia.
Possible uses of black magic/darkness in the 12 (just throwing these out there)
We know that Zeref uses black magic. He's able to summon this magic with no real change to his body, though his eyes do change somewhat when he gets angry. If Gray's Black marks are some what related to darkness/black magic, then we see his skin going black. So with these in mind, here are some
possible displays of darkness/black magic...
August: Is this him using his darkness?
God Serena: Is darkness used to harness the power of 8 dragon slayer lacrima?
Neinhart: Remember these eyes? (lol)
DiMaria: What is Ultear implying here? That DiMaria did something
dark maybe?
Awakening Darkness
Invel said this that Gray's darkness will awaken after he kills Juvia. He also said that his slayer magic is eating at Gray's soul, and that the darkness is on the verge of spilling out. So how did the S12 awaken their darkness?
- Irene did something terrible in Isghal, or something terrible happened to her
- Neinhart perhaps a history tied with Irene's/Erza's and it's a deeply emotional one for him
- God Serena... nah he just wanted more power

- Brandish with her mother being killed, and maybe Zeref told her that Layla killed Grammi.
- August, Larcade and Invel... I think they were able to awaken darkness on their own under the tutelage of Zeref (ie no emotional trigger)
Jacob had a really bad experience with a girl
Darkness as a magic concept
1) In Tartarus arc, Mard Geer said that curses were a new path for magic, fueled by negative emotions. He also said that Gray 'demonised' when he used the Black markings against Momento Mori. Perhaps darkness and curses are a similar form of magic. This is supported by the idea of a heart being dyed in darkness which in turn grants fearsome power... sounds similar to the notion of negative emotions fueling curse power.
2) One of the few times magic (One Magic in particular) was discussed was on Tenrou when Hades was defeated. Precht, someone who was taught the basics of magic by Zeref, said that the true nature of magic is darkness...
Precht might not be right. But his knowledge of magic is almost unrivaled. Adding in Invel's statement (a more new and relevant one I guess), then darkness is likely a path of magic. Similarly, light can be a path too...
So how are FT able to fight them?
Invel said that darkness is in everyone's heart. And we know that through this whole manga, emotions and magic power have a connection. So if darkness can be a fearsome power (as an emotional driver for magic power), then
light can be considered another way path of magic. This is probably obvious anyways, when there is dark in a narrative, there is light to balance or oppose it. So the Spriggans have darkness, and FT has light/friendship/PoF (whatever).
This is to a degree supported by Fairy Tail being stronger than most (inb4plot). It's not just FT, but anyone who embraces light/bonds/nakama type stuff. At the GMG, Mavis knew Jellal wasn't a FT member, but could sense his
soul was the same as the guild members. So this at least supports the idea that the guild members
souls are somewhat special, because they're dyed in light.
It should be noted that FT/good guys have produced wizards of comparable strength to the Spriggan 12. So if light is more or less the same as darkness in the context of magic, then it's quite likely that they yield similar results as a magic power.
Fairy Heart
Also, when Lumen Histoire is first mentioned, we have Ivan calling it the darkness of Fairy Tail. The Mavis calls it light. This points out 2 things... 1) That there is light and darkness in terms of magic, and 2) depending on perspective, one might see it as darkness and the other as light.
In conclusion, the 12 use Black magic or walk the path of darkness, and this is why they're so strong magically. FT and others from Isghal walk the path of light. The simplest way to view it is to look at Zeref and Mavis on design alone.