The abyss continues the tower from floor 51 after you conquer the main tower. You encounter mutations that change combat conditions and earn prestige for defeating new floor bosses.
Select an abyss floor in "Tower" and inspect its mutations in "Scouting Intel". To review a mercenary’s relic, open their sheet under "Mercs" and choose "Gear".
Read the floor conditions before sending the party that conquered the main tower into the abyss. Bring mercenaries wearing relics that counter those conditions, and check remaining health between battles before continuing the climb.