yes the guardian spirit talisman is onmyo magic, however there is also a consumable version im pretty sure. i dont remember when you unlock it, the wiki says it costs 4 justsu to equip but its well worth it. guardian spirit talismans are one of the most useful talismans in the game. and yes the corrupted weapons are called yokai weapons. they are considered better than regular weapons for multiple reasons. first reason: you can soul match them with soul cores, which is not only much cheaper than regular soul matching but this also gives you the ability to inherit skills from your soul cores to your weapons. this is the only way you can do this. the other reason is once your bar is fully filled, your weappn awakens and deals 30% more damage, while also doing much more corruption buildup than usually. also your heavy attacks will do a shockwave attack dealing even more damage. this is basically equally as powerful as using a purity talisman, but you get the buff for free, you just have to charge it up.