The Underworld's Troops and methods of earning them are are different from Krystara's Troop cards.
In the Underworld, instead of Kingdoms, there are Factions, each with their own set of Troops and unique Chaos Portal.
Chaos Portals are like Chests for the Underworld. Underworld Troops are not found in Krystaran Chests. Although Troops from the Underworld are exclusive to the Faction that their Chaos Portal is from, Chaos Shards can be used across Factions to open any Portal.
Opening a Chaos Portal
Open Chaos Portals using 20 Chaos Shards which can be found in Delve Chests and Faction Events. Select a Faction that has been unlocked, then select the "Portals - Troops & Treasures" button. Portals also have a chance to unlock Treasure. Like normal Chests, neither a Troop or a Treasure is guaranteed when opening a Portal, and there is luck involved in what you will receive. There is a higher chance to receive Faction Troops over Faction Treasures in Chaos Portals.
An example of Treasure and Troop rewards from opening 10 Chaos Portals.
Faction Troops are earned by opening Chaos Portals. Faction Troops only drop from their Faction's Portal. Underworld Troops have special Traits which make them more powerful in Delves, such as "Deep Strength - Gain 7 Attack when attacking Delves."
Underworld Treasures you find from Delve Chests can be added to your Treasure Hoard. They can also be collected as Troop cards to use building Teams, and have Spells and Traits related to gaining Gold. Like normal Troops, they also have different Rarity levels.
NOTE: If you spend all your Treasure on upgrading a hoard you will also spend all copies of that Treasure Troop card.
A Treasure Troop card: King's Crown showing its unique Traits.