Deathweed

Deathweed was introduced in the 1.0.5 patch.

It appears as a short double stalk of corrupted grass with sickly green bulbs at the ends. It grows in The Corruption, on ebonstone, and can be cut by virtually any weapon or tool.

Deathweed is an ingredient in 4 different types of potions*:

Magic Power Potion - 1x Deathweed, 1x Bottled Water, 1x Moonglow, 1x Fallen Star

Battle Potion - 1x Deathweed, 1x Bottled Water, 1x Rotten Chunk

Thorns Potion - 1x Deathweed, 1x Bottled Water, 1x Cactus

Gravitation Potion - 1x Deathweed, 1x Bottled Water, 1x Feather, 1x Fireblossom, 1x Blinkroot

blooms during the blood moon, at this time it will drop 1-3 deathweed seeds when cut.
 * You can see this listing in-game by placing Deathweed in the Guide's Crafting menu.