Crimson Seeds

You plant these to build a crimson patch deliberately, almost always to farm what only crimson produces: vicious mushrooms for powder, and a nearby source of ichor instead of a hike across the world. It spreads outward from whatever you seed, so dig the containment trench first. Doing it afterward means chasing the biome through your own basement.

If your world rolled corruption the Dryad will still hand these over, but only while she is standing in a hardmode graveyard. Throwing up a few tombstones next to her house is a couple of minutes of work and it is the cleanest way to start a crimson pocket in a corruption world. The eye is the other route, and it is far less reliable.

Dropped on mud rather than dirt these produce crimson jungle grass, which is the base for a jungle that is also infected. That combination is the point on some farm layouts and a genuine problem if it happens by accident anywhere near your real jungle, so check what you are standing on before you scatter a stack.