Evolution Core

Evolution Core is item used to transfer old-style pet to new style partner that can learn skills. Pet needs to be leveled to level 10 by feeding it enchanting materials or crystals or more expensive pet food.

Evolution has 20% change to succeed (not officially confirmed). Pet's level decreases on failure. With patch v103 Evolution Core should give 100% change to get a partner. Using Evolution Cores save you a lot of Legacy Essence.

Evolution Core is crafted using Alchemy.

Legacy information
Most players usually advise against using them due to low success rate. It takes a lot of resources and Production Points to craft a single Evolution Core, and buying them is also expensive.

Since the method of combining two Grade 10 pets to acquire one Partner seems to have 100% success rate, it is usually recommend that you avoid Evolution Cores unless you acquire them from an event. You can also get a good amount of Legacy Essence and Pet Snack from Partner Adventures, which makes it easier to acquire partners by combining pets.