Complete all Underworld Encounters.
There are 4 Invasion from the Underworld cards. You get the first on automatically by finishing a game after the release of the Title Update 2. They are easy to recognize because of the red banner and icon in the top right hand corner of the card.
Quick Guide to Underworld Cards:
Indecent Arrival: Follow. Interrupt. Kill the Hell Beasts.
Alicia in Underworld: Pay 3 gold for a 25% success chance (You can repeat this). Give Artefact in your possession. Offer to Find Mystics. Kill Hell Beasts.
Greenborough Forrest: Pass 50% Success chance. Chop (You must have a strong weapon in your possession). Move to Underworld Totem card. Give 3 rings.
Descent into the Underworld: You must win a series of battles where you need to keep returning to the card to face the next battle.
Battle 1: Hell Beasts
Battle 2: Hell Beasts and another Enemy card (I saw Minotaur or King of Scales here)
Battle 3: Hell Beasts and a Lava Golem
Battle 4: Hell Beasts, 3 Lava Golems, and a Lich
*If this solution helped you in some way, please vote positively. If you think it needs improvement, please let me know how in a private message or by leaving a comment.*