Soul Eggs are a type of Mystical Egg primarily gained from Prestige that grant a persistent permanent 10% boost to all future earnings, which can be raised further with Soul Food and Eggs of Prophecy.
They resemble stones with a purple five-faceted jewel set inside.
The Soul Eggs you earn when prestiging are calculated based off of your total prestige earnings, via the following formula.
(prestige_earnings / 1,000,000) ^ .15) * epic_soul_egg_prestige_bonus
Your prestige earnings can be found under Stats.
There are a few ways to increase the power of Soul Eggs.
Soul Food Edit
Soul Food, an Epic Upgrade, strengthens the ability of Soul Eggs. Each level adds a 1% boost to all Soul Eggs (e.g. a level 4 would have a 14% boost). Soul Food has 140 levels, with a maximum of 150% bonus to egg value per Soul Egg.
Eggs of Prophecy Edit
You can increase the potency of soul eggs by 5% (compounding) with Eggs of Prophecy. Each Prophecy Egg increases the potency of all soul eggs by 5-10%, depending on your level of Prophecy Bonus, an Epic Upgrade.