Crimson Seeds

Crimson Seeds are a type of seeds which can be planted on Dirt Blocks. If done so, they will spread the Crimson in all directions through Crimson grass. They can only be obtained in a Crimson world, either as a drop from the Eye of Cthulhu or purchased from the Dryad during a Blood Moon.

Trivia
Before 1.2.3, You can buy Corrupt seeds from the Dryad, on a Blood Moon on a Crimson world. People used this to their advantage.