To create seeds for mobs, you need to collect souls. This can be done in multiple ways.
Souls are collected using Soul Jars. Each Soul Jar can hold the souls of a single mob type. The amount of souls required to fill a Soul Jar differs depending on the mob.
The Soulium Dagger is a special weapon that you use to collect souls from mobs. The basic Soulium Dagger gives 1 mob soul per kill.