A few days ago Wizards published the December issue of Unearthed Arcana.
Let's go to review it.
The monthly theme is Black Magic (old Black magic title says) which for a rpg like D&D is always a good thing.
First of all the Tiefling race is slightly modified and divided into two subraces, according to character's infernal heritage: fiend of Nine Hells (Infernal Tiefling) or demon of the Abyss (Abyssal Tiefling).
I must admit I'm not a great Tiefling fan so judgment of mine may be not truly objective but even if I appreciate their try to have different kinds of Tiefling (like many other races) I'm not fully satisfied by Wizards' choices.
Infernal Tiefling is very like the standard one in PHB while Abyssal shows new traits.
The main Abyssal's trait in my opinion gives nothing to me related to Abyss. The ability to cast random minor spells, while surely caotic (and then related to demons) seems to me much more a Sorcerer feature (related to wild magic).
I'm not sure if I'd allow those subraces IMC.
The four new conjuration spells are very interesting.
They fill the gap about the lack of many conjuration spells of previous editions and with the DC check and ST every turn (to force the creature under caster's will) and the chance to attack the caster or his allies, they give the feeling about how difficult and risky is to summon demons and have them to do what you want.
One of them will be very useful IMC too.
Even if UA says those spells be only in Sorcerer and Wizard lists I think they can be also in Warlock's, expecially if she has chosen Fiend or Great Old One Patrons.