This month's Unearthed Arcana is on the small side, but it punches well above its weight in the Rule of Cool. Technically, all that is here is a new subrace and five new spells. On the other hand, it's breaking tieflings down into subraces - a welcome change - and breaking major boundaries on how Conjure spells work. As someone who does an immense amount of homebrewing design, I often feel like UA articles serve as much to point out new possibilities to third-party designers as any other end.
Now, as every schoolchild knows, the Player's Handbook tiefling has no subraces. The race has abandoned the story of Bael Turath and their kingdom-wide corruption by the Infernal powers, returning to the 2e and 3.x-era origin of the race. This article redefines the Player's Handbook tiefling as the infernal tiefling, and tags their Intelligence boost, hellish resistance, infernal legacy, and Infernal language as traits of the subrace. By contrast, then, we see an abyssal tiefling, who should reflect the nihilistic chaos of the Abyss and the brutish power of demons.
Correspondingly, they receive:
a Constitution bonus in place of Intelligence. Sure, that seems right for demons.
Abyssal Arcana grants spellcasting just like infernal legacy does, but your spell options each day are random. As you might expect, it's a cantrip at 1st level, a 1st-level spell at 3rd level, and a 2nd-level spell at 5th level, but you determine each spell randomly after each long rest, and you definitely cannot have the same spell two long rests in a row.
The cantrips specifically avoid attack options, which is good because a full change in playstyle would teach players the wrong lesson in how to approach this ability.
The first-level spells are all pretty high-functioning, including damage, crowd-control, and healing. Healing feels really weird here, because one wouldn't usually associate the Abyss with healing (or, if you do... why again do fiendish warlocks not get cure wounds?). It doesn't break anything, but I wonder what it's doing here.
Also, the salient spellcasting ability is presumably Charisma just like infernal legacy, but the text does not specify.
The second-level spells are a spread of utility effects.
What I see here, then, is that none of the cantrips is the "best" one - they're all okay. Likewise the first-level spells - they're all very desirable. The second-level spells too are useful and going to change your personal tactics for the day, but more toward avoiding than engaging combat.
Abyssal Fortitude matches up to hellish resistance, providing a minor hit point boost rather than resistance to a very common damage type. Ten hit points at 20th level is useful, not exactly a big deal. This could probably be one hit point per level without breaking much, though their Con bonus helps here as well.
They speak Abyssal, of course.
Other than the thematic note for Abyssal Arcana, this is basically fine and good. If I were going to play a tiefling, this is just as interesting as the infernal tiefling. I'd throw both of them over for a daemonic (yugoloth-derived) tiefling - of all the Blood War's combatants, I find the yugoloths the most interesting just because they've received the least attention over the years.
In principle, there is essentially no way for me not to be a fan of new spells, even mind-breakingly complicated ones... since I tend to write terribly complicated ones myself. In this case, we have five new spells that all start with the word Conjure. They each allow you to conjure one type of demon, but unlike other conjure spells, you may or may not have any control over what happens next. Unlike the UA document, I'm going to talk about them in order of increasing spell level.
Just one more note first. None of these have been added to the warlock spell list. Instead, the wizard and sorcerer got all five. This is almost certainly because they worry about warlocks ruining the flow of gameplay by using complicated conjurations all the time, so instead they'll package these as invocations to make them once-per-long-rest only and more costly to the warlock. It's one of the problems with the warlock as a whole.
Conjure Lesser Demon (third level) calls up a whole pile of dretches or manes, using a vial of fresh blood from a slain humanoid. The vial of blood from a humanoid killed within the last 24 hours is a running theme among these spells, though only two of the five to use it as a required material component. You can get around that with an arcane focus... which might be smart, because the vial has an alternate function within each of the spells, and that function uses up the blood.
Specifically, in all five spells, you can and really really should create a circle around yourself with the blood from the vial. Anyone standing in that space (which might or might not be you - beware of forced movement) cannot be targeted by the conjured demon(s). Thaumaturge-General's Warning: In addition to forced movement, look out for demons with area-effect attacks that do not have to target a person... like the vrock's spores and stunning screech, the hezrou's stench, the barlgura's entangle, or the dretch's fetid cloud. Short version: these spells are exceedingly hazardous to your health, and we haven't even gotten to the part where you can lose control over the demons.
One available reading of the guidelines for the demons' attacks relies on the technical definitions of the word attack in D&D. If you think these spells are using the word ultra-technically, then none of the demons get their area-effect powers, because none of those are attacks - they force saving throws, and many of them are casting a spell.
Anyway, dretches and manes aren't especially terrifying even in large-ish numbers, though they're useful for accomplishing things in poisonous environments, or for when you want to drag the flow of combat to a screeching halt as either you or the DM roll for all of their attacks. (An eighth-level spell slot gets you 32 of the bastards, which should be enough to trigger an aneurysm in your DM.) Even if you don't inscribe a circle around yourself, these lesser demons always go away when you lose Concentration on the spell, and have no way to break free of that requirement.
Conjure Barlgura (fourth level) is where things start to get interesting. This spell has a duration of 10 minutes... not Concentration. If that's not enough to make you nervous, you aren't paying attention. The only control you ever have over this creature is that it has a pretty clear-cut aggro meter: it always attacks the nearest non-demon that it can perceive. Conjure it right up in your enemy's business, and tell any allies you have to stay the hell away. Also hope your enemy cannot teleport. It keeps working its way through "nearest non-demon targets," choosing randomly if two creatures are equidistant. Don't use this spell unless a) all of your allies are already down or b) you have a good way to get rid of the barlgura. Casting this spell right before teleporting your whole party far away is promising, too. A CR 5 creature for a fourth-level spell slot is pretty serious.
Conjure Shadow Demon is also fourth level, and a lower CR than the barlgura, but comparatively speaking you get a lot more control over it. The shadow demon has a more complicated attack pattern, with a chance (based on your Charisma... sorry, wizards) to break free of your control whenever it cannot attack anyone on its turn, or when it ends its turn in an area of bright light. I always like it when lighting conditions matter in combat, because I so often forget to take them into account. Beating a shadow demon (Cha 14) in a contested Charisma check isn't reliable even for sorcerers. Anyway, as long as you can keep it satisfied with targets to attack, this spell is very potent. If the demon does break free, the spell no longer has a Concentration duration, though it does still have a one-hour duration limit.
Conjure Vrock (fifth level) gets you what has always been my favorite of the tanar'ri, in part because I have such an awesome painted mini of one It also brings in more themes of sacrifice to keep the demon sated, with the potential for a lot of mid-combat negotiation. This is a CR 6 demon for a fifth-level slot. The spell's consumed material component is a gemstone worth 100 or more gold pieces, and in this case there's a real incentive to use a more precious stone - its market value grants you a bonus to your Charisma check to keep the vrock under control. Even better, if it leaves your control, it goes to the next highest bidder in a telepathic auction. Vrocks are fast, pretty tough, and especially useful for their two special abilities... again, just keep well clear of them.
Conjure Hezrou is the big finish, a seventh-level slot for a CR 8 creature. Comparable to the vrock's desire for gems, the hezrou wants food - 100 gp worth of it or more. If you fail to control it - and the DC of that check is going up every round, so you're going to fail unless you end your Concentration on it early - it starts eating the dead, or making more things dead and then eating them. The good news is that it returns to the Abyss if reduced below 50% of its maximum hit points. Anyway, hezrou are great melee brutes - solid AC and hit points, and three attacks per round. Just hope that your enemies focus fire on it - but I suspect that even at 13th level most enemies can't just ignore it for the four or more rounds it takes for your Charisma check DC to get untenable.
I love this playtest document. I love the new conjuration mechanics most of all - circles of blood, sacrifices of riches, it's great stuff. The abyssal tiefling is also a welcome variant, showcasing a somewhat complicated but definitely interesting racial ability. That's actually the theme of the article: more complicated at the table than a lot of the content they release, but with compelling payoff for the time that it takes. Let's be real: no one is going to remember the details of conjure hezrou without looking it up, but it doesn't matter because it probably won't happen that often.
Also, look for a magic item, prestige class, curse, or something that changes your Type to demon...