Corrupt Seeds are 种子 which can be planted on 土块s and will then spread 腐化之地 in all directions. The corruption will only spread above sea level. They drop from the 克苏鲁之眼 and can be purchased from the 树妖 for 5 during a 血月 in a 腐化 world.
Corrupt Seeds can be used to 农场 魔菇s and may be used to spread Corruption in Crimson worlds. However, Corrupt Seeds cannot be obtained within crimson worlds and must be brought over from a corrupt world.