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.
To increase the soul siphoning efficiency of the Soulium Dagger, you can upgrade it to have either a Hostile or Passive attunement. This is done using the Infusion Altar.
An attuned Soulium Dagger will provide an additional 50% souls gathered from every mob that applies to the attunement. They also gain a slight damage increase.
The Soul Extractor is a machine that takes in mob drops (such as Rotten Flesh, Gunpowder, Bones etc.) and converts them into a certain amount of mob souls.