The Souls ($SLS) are very rare stones in the Underworld. They are said to contain incredible powers. Pets and EvoSkulls can utilize them to increase their experience.
$SLS is needed for all sorts of things in the Underworld, increasing the level of the Pets and EvoSkulls, purchase mystery boxes from the merchant and more coming in the future (Enchanting Gems, access to some special quests and future gamification features).
You can earn $SLS by sending your level 1 or above Pets/EvoSkulls on missions. Pets also earn 1 $SLS daily per pet. (Accumulates for up to 7 days). In addition to that, each CroSkull will earn 1 $SLS passively while on their Adventures every 30 days of not claiming.
Here's how:
The amount of $SLS your pet earns depends on your pets level, and how many consecutive missions you send it on. You can find the chart on $SLS earnings per level here.
Each CroSkull earns on their Adventure 1 $SLS for every 30 days spent adventuring (Claiming $GRVE resets this 30-day counter). To get $SLS you need to have your CroSkulls staked when the 30-day mark has passed. (When Malus reaches 5%).
Some raffles reward $SLS as prizes. In addition to that, in the future, you will be able to earn $SLS through CroSkulls Rivals. Use this opportunity to get a headstart, so that you are ready for battle when it releases!