King of Avalon almost always has one or two seasonal events being active. At this time the Plague Prevention Event is one of them. This guide explains how the event works.
During the duration of the event you receive Tainted Daggers when you kill monsters and rally barbarian camps, and Dewdrop Herb when you are gathering resources. These can be synthesized into Cleansed Daggers, which can be used for rewards.
You can get Tainted Daggers by killing monsters and successfully rallying barbarian camps. So far I typically only got 1 Tainted Dagger each time I kill a high level monster or rally a high level barbarian camp. I have not tried lower than Level 28 for either.
You get Dewdrop Herb when gathering resources. The amount differs quite a bit. On level 6 tiles I have received between 2 and 6 Dewdrop Herbs, On level 7 tiles 6 or 7 Dewdrop Herbs.
I have not yet checked whether I also receive Dewdrop Herb when gathering on alliance resource tiles. Typically this is not the case during events.
You can Synthesize 3 Dewdrop Herbs and 1 Tainted Dagger to one Cleansed Dagger. If you are an active gatherer and hit highest level monsters and barb camps, you likely have a lot more Dewdrop Herbs than you can use.
You get your prizes by selecting the Cleansed Dagger items in your items and clicking Use. Prizes I have received include: Gold, Intensity Crsytal (I), 100 Lord EXP, 100 Dragon EXP, 100 Dragon Spirit EXP, 10 Assault/Guardian Power, 30m VIP 1K Food/Wood, 200 Iron, 50 Silver, Knucklebone, Summoning Horn, 5m Speedup, 60m Speedup, 2h Firm Defense, Materials (Animal Hide, Cerussite, Pine, Bronze, Horn, Amethyst, Spider Silk), Stallion Stone (I), Refining Rune (I).
There is not much to it, just do what you normally do and collect the extra prizes, the same as several seasonal events before. If you have any questions, or comments, let me know in the comment section.