Atlantis (Season II) and Event Heroes belong in different family groups, and similarly, the Valhalla (Season III) Heroes belong to different Realms. These families and realms provide bonuses based on how many unique Heroes from the same Family or Realm are present in a team.
The Bonuses or effects of the Family and Realm groups can be viewed by tapping the Family/Realm symbol on a Hero's card.
Families give bonuses for each unique Hero of the same family in is a team. There must be at least two unique Heroes from the same family for the bonus to be active.
Realms empower their members in different ways. The more unique Heroes of the same Realm there are in a team, the more powerful the Realm Effect. The Realm Effects are somewhat different from the Family Bonus, in that, they are active, even with just one Hero of that Realm in the team. For each subsequent Hero of that Realm, the effect increases up to a certain amount. For example, in the case of Helheim Heroes, with just one Hero of that Realm, 25% damage will be dealt if they die, with 2 Heroes this increases to 50% and with 3 Heroes it will be 100%.