Just what it says on the tin. Mirror perk progression is extended, every single one of them has more ranks, and almost all ranks are cheaper (although some of them aren't as powerful at the start, all of them definitely very powerful when maxed), ensuring you can buy at least something, even if you die as soon as reaching the Fury the next escape attempt. Most perks now have 15-20 ranks, although there are a few over 50.
That said, maxing out the now-OP perks won't be quick: the total sum for maxing out both perk trees exceeds 200000 Darkness (although for peak combat efficiency that'll be more like 100000). But I tried to make sure that reaching the same median power level as with maxed out vanilla Mirror comes at about the same Darkness expense, around 10000. If you want to max it out, but aren't a fan of korean-level grinding, there's an extra file that increases bonus for using the designated weapon from +20% to +100%.
Oh, and the key cost of unlocks is changed: instead of 5/10/20/30 as per original, they are now 2/4/8/16

This mod comes in two variants: "old format" and "new format". The following installation instructions are for the "old format" variant. The "new format" variant requires ModUtil and the instructions are provided on the tool's page. There is no difference between the two otherwise, and please remember to only use one of the two, not both.

How to install:

• Reset your Mirror perks! Because of the change to the rank costs, it will cause a discrepancy between what your savegame card says and the real amount of Darkness you have.
• Locate the installation directory. On Steam, it's done by right-clicking on the game, selecting "Manage" and then clicking on "Browse local files".
• Go to Content, then Scripts
• Move the original MetaUpgradeData.lua file from this directory somewhere safe or rename it.
• Put the MetaUpgradeData.lua from the archive at the place of the original.
• Optionally, repeat steps 3 and 4 for TraitData.lua. This is not required for the mod to work, it's simply that extra which makes farming Darkness easier.
  

How to uninstall:

• Reset your Mirror perks! Otherwise the save may not load when the mod is uninstalled.
• Repeat steps 1 and 2 from the installation process above.
• Delete MetaUpgradeData.lua (and TraitData.lua if you used it) and move the backup(s) back into place. Alternatively, you can redownload the fresh original file(s) from Steam: right click on the game -> properties -> local files -> verify integrity of the game cache.
  

Known issues:

• Having more than 2 Defiance charges prevents you from getting the Defiance boon. If you are still yet to get it for the prophecy, don't level up more than 1 rank, or the boon will stop showing up in Chaos gates.
  

Warning: the next game update will most likely overwrite these two files, returning the game to vanilla state and most likely making the game refuse to load your saves. In this case, you can try and reinstall the mod, but if the new build changed the original files, there may be crashes and some other weirdness. If that happens, message me (preferably in Discord) and I will roll out an update.