Overview:
The modification changes the basic Triglav Runestone for one that has a guaranteed chance to instantly kill the enemy.

Vanilla version Triglav Runestone
Lesser 2%|Normal 3%|Greater 5% Chance to Stun

First version of my mod
Lesser 2%|Normal 3%|Greater 5% Instant kill chance

Second version of my mod
Lesser 10%|Normal 15%|Greater 25% Instant kill chance

Third version of my mod
Lesser 100%|Normal 100%|Greater 100% Instant kill chance

Fourth version of my mod
Lesser 100%|Normal 100%|Greater 100% Instant kill chance + ignoring the resistance to instant kill (1 hit boss(tested on first gryphon))

Installation:
Download with NMM and simply activate mod.
or
Download manually and unpack "modInstant" folder to Witcher 3 main directory/mods folder. (example D:\The Witcher 3 Wild Hunt\Mods)

NOTE:
I used
xml.bundle from The Witcher 3 Wild Hunt\content\content0\bundles,
damageManagerProcessor.ws from The Witcher 3 Wild Hunt\content\content0\scripts\game\gameplay\damage
Might be a conflict when other mod uses def_item_upgrades.xml or damageManagerProcessor.ws
If conflicts try:  Script Merger by AnotherSymbiote 
I tested this modification only at the lowest level of difficulty, NG and on 1.21 version.
If you have any problems, please let me know.

Compatibility:
I recommend my other mod Better Ciri Sword <If conflicts try:  Script Merger by AnotherSymbiote>

I inspired:
RuneEnhanced by zxdong262
Instant Kill Rune By: thril1
Enhanced Runes By: thril1
Ultra Gore 2 - A Dismemberment Mod by Yoseppy Armineppy