Reference

Master Slime

Master Slime The ability: Master Sacrifice I, Grants 15,000 Skill EXP to Heroes when sacrificed.

Slimes can’t be used in battle. Instead, they’re used to increase your Heroes’ skill levels.
Hero Altar – Consume any Sacrifice

Master Sacrifice I

Claim

Hire more powerful Heroes at the Hero Altar!
Switch your Heroes Altar to Item Mode to store hired cards and essences in the Warehouse.
You can hire Heroes by spending Shards, Gems, or Honor.

Hire Hiring Cost Chances *
Hire with Shards 750 Shards 100 %

* -Chances are approximate.