It seems that when an attack involves base damage such as slashing and an extra source of damage, such as poison damage, that requires a save, both damage sources are considered to be instantaneous and unified for the purposes of how much damage a PC will receive, and as such will be applied at once.
My particular question involves a situation such as that posed by the death panda known as the Zorbo. The zorbo has a feature that when they hit, the target must make a dexterity saving throw, and on a fail, takes a permanent and cumulative -1 penalty to an item that improves its AC.
My question is if the PC is knocked unconscious by the hit, what order does the dexterity save trigger:
a) The PC would take the damage and as part of that "overall attack," make the dexterity saving throw as they would normally, then go unconscious.
b) The PC would take the damage, go unconscious from said damage, then as part of the unconscious condition automatically fail the dexterity saving throw.
Best Answer
The problem
The Zorbo's attack does the following on a hit:
There are thus two primary consequences of being hit by a Zorbo; taking damage, and making a save. If the damage reduces the target to zero hp, there is the further, secondary consequence of falling unconscious, which includes going prone and applying the unconscious condition. Thus a single hit can generate at least four consequences, and some of these consequences can be affected by the order in which they are applied - as you point out, the Dex save is automatically failed if the unconscious condition has been applied before the save is made. Order matters.
What the rules say - and don't say
The rules on Making an Attack say:
Step three, resolving the attack, contains all of the guidance we are given in how to apply simultaneous consequences of a hit - that is, practically none. "Some attacks cause special effects in addition to...damage" is all we have to work with. In particular, the 5e rules are very much unlike a game like M:tG, where simultaneous effects have a precisely defined priority that determines their order of resolution.
Like many aspects of 5e, it then becomes the job of the DM to decide the order in which to apply the multiple effects of a hit.
Solars and Slaying Longbows
The Solar has an attack called Slaying Longbow which does:
Since the damage from the hit itself could reduce the target to fewer than 100hp and thus force the save, Jeremy Crawford was once asked the order in which to apply the consequences of the hit. His response was:
At first blush, this seems pretty similar to the Zorbo situation, so if you want to take this as RAI you can; first apply the damage, and the consequences of the damage including unconsciousness, and then the save. However, it is important to be aware that Crawford's tweets are not official. Further, even if you accept this it is unclear whether it is meant as a specific statement about how the slaying longbow works or a general statement about the order of simultaneous events, or even something in between like 'apply them in the order required to make the save happen, but not an automatic fail.'
An optional rule
Given the lack of an official rule in the ordering of resolution of simultaneously-triggered events, it is unsurprising that there is an optional rule, as presented in XGtE:
In general, this is a pretty good practice. It can resolve any number of simultaneously-triggered effects. It gives more agency to players on their own turns, while being fair to everyone involved. It can usually align with supporting the narrative. In your case of the Zorbo, if the Zorbo's attack happened on the Zorbo's turn, the monster would clearly want the save to automatically fail. In that moment the monster has the initiative, and overwhelms the character before the character can respond. On the other hand, if this happened on the player's turn, where the Zorbo's attack was a reaction, it makes narrative sense that the character with the initiative darted away from the Zorbo, was wounded, but maintained consciousness just long enough to attempt the save before succumbing to their wounds and falling unconscious.
Where I would not use Xanathar's
In certain situations, a strict application of the optional rule in XGtE results in diminished verisimilitude. Consider the poisoned crossbow bolt of a drow, which has:
Suppose a character is hit by a bolt on the character's own turn (such as when the drow had a readied action to fire upon a certain trigger). In this case, a player allowed to use the XGtE optional rule and order events as they chose could declare that they wanted to make the save first, and then, if they had been poisoned into unconsciousness, apply the damage in order to wake themselves up from the same hit that knocked them out. Clearly this goes against any reasonable application of the narrative, even if the character has initiative at that point. Thus, I would say that while the XGtE rule is generally good, DM's should feel free to override it when it conflicts with narrative verisimilitude. Fortunately in your case of the Zorbo, that doesn't appear to be an issue.