That Old Black Magic

From the introduction:

As the Rage of Demons storyline season continues to boil over in the Underdark, this month's Unearthed Arcana offers new demonic options for your campaign.

First off, this article takes a new look at the tiefling race. By breaking down the Player's Handbook tiefling into a modular variant, players can now create tiefling characters tied to either a devilish or a demonic ancestry.

Additionally, a new suite of conjuration spells allows casters to call forth demons to do their bidding. Each type of demon has its own proclivities and quirks when summoned to the material world, and demons are far more difficult to control than other creatures a spellcaster can call on.