Other than the prompt of “It seems to want something”, it’s not clear what you’re meant to do. We’ve seen people trying to purify them with ki pulses, attack them, throw things at them, but all you really have to do is give them an item to trade. It’s also worth noting that the purple evil Kodama do not count towards the list of hidden green Kodama in any given level.
What to do with Evil Purple Kodama in Nioh 2
To repeat, you do not have to purify or cleanse the purple Kodama to make them into their green cousins. The “something” the evil Kodama want is an item from your inventory. Once you’ve interacted with enough of them to learn their language, it becomes obvious they don’t even want to inconvenience you with their request - they just love shiny things. To play ball, you need to go into your inventory and press Triangle to drop an item. The Kodama will then pick it up and reward you with a similar item of the same level and rarity.
This can be a weapon, a piece of armour, a soul core, or a consumable. In my experience, if you give a Kodama a soul core, it will give you another of the same kind, whereas weapons and armour are usually different. This is a good way to swap out useless pieces of equipment that are high level and purple rarity. For example, if you only use light armour but have a ton of purple, heavy greaves, you can give them to the evil Kodama for a change at something more useful. Occasionally, a purple Kodama will ask for the smelly round thing - a dung ball - from your inventory. In return they usually give you a ton of rare consumables. For help with the basics of Nioh 2, here’s our combat tips page - including how to pull off a perfect Ki Pulse. Alternatively, we also have pages on the best weapons we’ve found so far in Nioh 2, as well as the best skills. Finally, here’s on page on how to get the most out of your Guardian Spirit and Soul Cores.