Also of note, The Slayer is not a nickname that humans use to refer to Frieren. This is an epithet that only Demons use to refer to Frieren.
Sancturillore -- Élie Simard-Wallot
2023-12-14 19:05:16 +0000 UTC
The reason Lugner said that demons surpassed Zoltrack half a century ago (so about 50 years ago) was because by that point demonkind had been fighting human mages using Zoltrack(ordinary offensive magic) against them for about 20-30 years.
While humans went with blocking Zoltrack with better barriers and with upgrades on the magic resistance of magic armors, demons went about it purely with magic resistance.
Demons already focused on making their own body more resistant to magic. And since the sealing of Qual and after facing humans using Zoltrack against demons for a while, demons found a way to boost the magic resistance of their bodies to block piercing spells like Zoltrack.
However, the version that Frieren uses is made to specifically go through this new magic resistance technique, and was thus not taught to most humans so that demons don't learn of it and prepare to block it until it is too late for them.
This is why Lugner was surprised when he got affected that much by Fern's Zoltrack, and how he figured out that it was Frieren's special modified version that is modified to work on demons.
We never learn (as far as I recall) how the demon-killing version actually harms demons more, but my personal guess is that Frieren's modified version might actually target the magic particles that demons are mostly made up of. Or maybe the method is different. Regardless, it does do more harm to demons and monsters than against other stuff (like humans).
Sancturillore -- Élie Simard-Wallot
2023-12-14 19:01:12 +0000 UTC