Some of the heroes in Valhalla Summons have their realm bonus explained if you touch the icon.
Some realm bonuses activate when they’re below 20% HP, e.g. Midgard heroes like Thor and Mist will give their allies a +5%/+10% attack buff until they’ve healed or die in battle.
Other realm bonuses activate upon the death of a hero from that realm, e.g. Muspelheim heroes like Sumle and Baldur will apply an uncleansable burn debuff onto the enemy team until they’ve healed or die in battle.
Not sure what language you play the game in, but if you ever curious and happen to have at least one hero from Valhalla/Season 3, open their card and touch the icon above “Realm” and it should explain that particular realm bonus.
I hope this is of some help (even if it might be repeating previous answers/replies)!