How do you get skill book cores and skill book ultimates in Heroes vs Hordes?

You can get Skill Book Cores and Skill Book Ultimates from three places in Heroes vs Hordes:

1. Faction Wars
2. The Rune Portal
3. As a reward in event exchanges

Both of these resources are very hard to obtain. They are used to level up a hero's skills after you increase the hero's star level. New heroes, in particular, get dramatic increases in their damage from leveling up their skills, making both os these skill books valuable.