To promote a hero you need Essence, a Level 25 hero and another identical hero to the one you want to promote.
Like Marauder L25 or more + Marauder any level.
heroes need to be identical both in name and in stars.
To promote an Epic to 5 stars you need 4 identical 3 star heroes. Like 3*+3*=4*, 4*+4*=5*.