Máttugr Helson is a Troll known as "The Bridge Keeper". He is encountered in Helheim and was given the responsibility of protecting the Bridge of the Damned and ensuring that all entities crossing the bridge are dead.
When Atreus falls ill after the fight against Magni and Modi, Kratos rushes Atreus to Freya for help. She states that Atreus' "true nature" (as a god) is fighting within him. She instructs Kratos that she needs a rare ingredient that can only be found in Helheim, and that ingredient is The Bridge Keeper's heart. Kratos must then traverse through the Realm of the Dead in order to defeat Máttugr Helson to obtain the heart and save his son.