From the introduction:
This month’s installment of Unearthed Arcana features a number of new character options ideal for an Underdark campaign.
Two new fighting styles for fighters, paladins, and rangers focus on close-quarters combat, whether defending allies or unleashing deadly ranged attacks at nearby foes. Both styles are handy for dungeon delving and for battles in the confined environments of the Underdark.
The Deep Stalker is a new ranger archetype, scouting out Underdark threats using a combination of cunning and magic. Deep Stalkers use magic and other tricks to infiltrate Underdark settlements and spy on their inhabitants. After gathering intelligence on growing threats to the surface world, they engage in hit-and-run battles to eliminate those threats.
The shadow sorcerer is a creepy spellcaster who commands the power of darkness. Though not always native to the Underdark, these sorcerers are often drawn to the dismal gloom of the world below.
Finally, the Undying Light is a new warlock patron. By forging a compact with the energy of the Positive Plane, a warlock vows to bring light into the darkest reaches of the world.