Unearthed Arcana plays a dual role in letting Wizards of the Coast preview new material for Dungeons & Dragons while also working as a bit of an open playtest for said material. In this instance, they're not waiting any longer for the Halloween season to start. They've got two new subclasses, one for Bard and one for Warlock, that tap into the spirit realm and the undead for their powers.
From the post:
Two spooky subclasses manifest in today’s Unearthed Arcana: the College of Spirits for bards and the Undead patron for warlocks. You can find them in the PDF below, and we’ll release a survey soon to find out what you think of them.