Risk of Rain 2 Purity Guide How It Works

Purity is one of the new lunar items added in the 1.0 update for the game. It is unlocked by beating the game on Monsoon difficulty. It can then be aquired ingame just like any other lunar item.

Now to fully understand Purity we need to look deeper into it and the wiki is a good source of info for this. On the wiki we can find both what lore it has but also its stats and how it scales. Luckily the item doesnt have any complex scaling like some other items in this game.

When you pick up the item ingame it has the following description (Reduce your skill cooldowns by 2 seconds. You are unlucky.)

The first part, the skill cooldown reduction part is quite simple, it just reduces your cooldowns by a flat 2 seconds and then scales by 1 more second per stack.

The other part of the item and what makes it so special is the unlucky part of it or the so called penalty of the lunar item. If we read what it actually does it says that it rolls luck based items one more time for an unfavorable outcome. If you have a good understanding of how the red item 57 leaf clover works then you will most likely have an easy time understanding this item.

What it means by it rolling once more is that you have to proc an item twice to get one proc. To explain this we can take a look at the item ATG Missile Mk.1.
The ATG Missile Mk.1 has a 10% chance of activating everytime you hit the enemy, The other stats of it arent important for this. Now what Purity does is it rolls that 10%twice and only activates the item if you hit that 10% both times. For an item like the ATG Missile Mk.1 this means that the activation chance gets changed like this:
10% = 0.1
0.1 x 0.1 = 0.01
0.01 = 1%

The ATG Missile Mk.1 now has a measly 1% chance of activation and if we look at an item with a different activation chance like a stack of 5 lens makers glasses which has a 10% chance of activating a critical strike per stack. Now normally you would also get a bonus 1% chance but to simplify this and make it easier to understand we will use 50% instead of 51%. Now how this changes with Purity is that the activation chance gets changed like this:
50% = 0.5
0.5 x 0.5 = 0.25
0.25 = 25%

By looking at these numbers we can clearly see a pattern that is a good way to just quickly keep track of what your “hidden” stats are in a run. The ATG Missile Mk.1 has a 90% chance of missing and its activation chance goes down 90% from Purity. The lens makers glasses stack has a 50% chance of missing and goes down 50% from Purity. This tells us that its the miss chance that the activation chance gets lowered with and that it rolls once more for every further stack you have of it.