A question about leveling up a hero and creatures...

This is a perk for the Haven faction in the magic of light, which you get when you level up, granting protection and blessing. When you use the racial prayer, you receive the Guardian Angel perk, which revives your strongest unit after all other units in that stack are killed. Additionally, if this unit is the last one to die and doesn't have time to retaliate, it will do so after being resurrected... It's a great perk, especially if you're planning to go into a Single Adventure map.