The Deathweed is a crafting material item that can be used to craft many necessary and helpful items that will make you significantly stronger. Here is a guide on how to find it.
The first way is going to the Corrupted Biome. You can easily spot it as it looks gray, ugly, and dead. It is a very good idea to collect these plants when there is a Full Moon, or better yet, a Blood Moon. Only in this time you are able to get the seed.
They can be very easily blended with the background. So make sure to pay extra attention to how it is slowly moving with the air. It is very small and transparent.
You can break them with any tool.
To get it into full bloom, you can wait for the Blood Moon, or simply summon it yourself by using Bloody Tear(only at nighttime). It should look like this.
Go ahead and just break away all the corrupted blocks. You can go crazy and use any type of tool to destroy blocks going downward. If you do not have Corrupted Biome, you can do this same thing in the Crimson Biome.
You will find Ebonstone Blocks. Collect some of them, and go ahead and place them anywhere in the sun on the surface. Once in a while, you will get a Deathweed randomly spawning here.
Just go ahead and use all the Deathweed seeds and plant them on blocks. You can make a farm with a line of Ebonstone Blocks, plant the seeds on them, and after a while, you’ll have your own Deathweed farm in your backyard. It grows mostly in the night time.
Go to the Dryad NPC and from here you can shop the Deathweed Planter Box. You can place these blocks and plant on them too.
I hope this helped out a bit. Good luck!