#Not necessarily, because math, and not at all in Adventure's League
##AL problems:
The key problem is :
We aren't using Wish to cast Simulacrum using the "replicate a spell" feature, since that would require being in range of Xanar to cast it. Instead we use the second option to wish for the simulacrum to be made no matter how far away he is. This isn't asking for much beyond the basic, and so should be a valid wish. This does incur the 33% chance to not be able to cast Wish ever again, but that's for your simulacrums and thus doesn't matter.
Because:
No Copies of a Copy.
Simulacrums can’t cast simulacrum, or any spell that duplicates its effects.
and
You Are You; and So Is He.
If a simulacrum you have created casts wish, both you and your simulacrum suffer the stress associated with casting the spell — including the risk of being forever unable to cast wish again. The inability to cast wish extends to any simulacrum you create in the future. (p. 8)
Each of which is designed to deal with essential parts of this otherwise-legal plan.
##General Problems:
- The first simulacrum uses the rod to cast Plane Shift, dropping the rod just before it leaves. The next simulacrum then takes it's turn, picking up the rod (interaction), casting Plane Shift (action), and then dropping the rod as well before it leaves (free action). In this way, the rod travels along the infinite line of simulacra, allowing all of them to cast Plane Shift.
You almost certainly can't do this. There's no time between casting and the effects happening. You could use Contingency for this (there are ways to have more spell slots around), except Plane Shift is too high of a level. Instead, you need to be friends with or True Polymorph into a Metallic Dragon variant spellcaster that knows Plane Shift ([for reasons][1]. You can also coerce one into service with Gate+Dominate Monster or whatever), and have your simulacrums using twinned wish duplicating simulacra. Despite the fact this makes no sense (Wish-duplicated spells shouldn't be twinnable since you aren't casting them), it is legal via Sage Advice Compendium errata stuff:
Can my sorcerer use Twinned Spell on a spell duplicated by the casting of a wish spell? And if so, how many sorcery points does it cost? Yes, you can. It costs the number of sorcery points appropriate for the level of the spell you’re duplicating.
and so by spending 7 sorcery points your simulacra can duplicate both you and the dragon (or a simulacrum of her), or you and a simulacrum of you.
Assuming you take one extra day or use up your 8th level slot or have a relevant magic item or class feature to cast Simulacrum again without using up your 9th level slot, that also solves the need to be using the cheat-y wish instead of the normal one: since the first casting gets you two simulacra and they each still have wish ready, the first one can copy you and the second one, and the second one can copy two of those copies, and so on. One out of every 9 copies needs to be a dragon.
You could have done this with Gynosphinxes instead, but you can't simulacrum those because they can't turn into a beast or humanoid while retaining the ability to turn back like Metallic Dragons can. Since draconic spellcasting has no components, the dragon simulacra can warp parties of 8 Xanar simulacra to the Abyss without the need for rods.
Next problem:
- Infinite simulacra of Xanar appear at every point in the Abyss, and cast an infinite number of Eldritch Blasts upon every demon there.
No, Infinite simulacra of Xanar appear at infinitely many points in the Abyss, and cast their infinite Eldritch Blasts upon infinitely many demons there. Your infinity and the Abyss are not guaranteed to be equal in size. In fact, the Abyss is probably infinitely larger: you have1 only ℵ0 simulacra while the Abyss has ℵ0 layers with probably ℵ1 points on each layer. It is possible,however, that the Abyss only has ℵ0 or some smaller number of points on each layer. In that case, this might be a larger number than the number of demons. Even so, there's no guarantee that your ℵ0 is the same size and their ℵ0. It might be bigger, smaller, or something else entirely. Comparing infinities is not easy, that's why ∞ - ∞ is undefined, not 0.
To solve this problem, you would need a system to generate an uncountable number of simulacra. That is, you would need the demons to not be able to apply Cantor Diagonalization to your onslaught and, in so doing, show that at least one demon wasn't attacked. I am not sure if this can be done or how you would do it.
Even if you did it, your sets would each be ℵ1 large, and you'd still have an ∞ - ∞ situation for your DM to define the result of. To get around that, you'd need to incorporate the source of the demons into your simulacrum creation, so that the number of simulacra can be shown to be greater than the number of demons at infinity and the result of the subtraction can be medium-well-defined.
You also need a mapping function to get your simulacra to not all be bunched onto the same points, but that's a smaller problem likely solved by assuming a 1-to-1 correspondence between Material Plane spaces and Abyss spaces, somewhat supported by the explanation of how planes work in the DMG.
Even then you are gonna miss infinitely many demons, though because of the next problem:
- Every demon takes an infinite amount of force damage, and dies.
Not necessarily. Demons in antimagic fields won't, and, in fact, no Xanars can warp into such spaces! So that's infinitely many demons this won't kill.
Also, you're rolling initiative and even with infinitely many rolls you won't beat any creature who rolls higher than it is possible for you to roll or who has special abilities allowing it to go first. Any creature who goes before you might take actions to elude your army, for example by casting antimagic field or plane shift or wall of force or meld into stone or doing anything else that negates infinitely many eldritch blasts. And any creature who doesn't go before you still might escape with reactions unless they are surprised (which, admittedly, seems likely, but is still not guaranteed). And those who don't might, like with antimagic field residents, be immune anyway for some other reason.
That said, this will still kill infinitely many demons, so it may well still be worth a try. Just watch out for the infinitely many survivors who immediately plane shift in on your position and give you a bad day.
- Okay, aleph numbers aren't numbers and you can't "have": ℵ0 of something. But linguistically that is a lot clearer and easier to read than "The cardinality of the set of your simulacra's serial numbers is ℵ0, whereas the cardinality of the set containing all points in space is probably ℵ1". This is an RPG answer, not a Math answer. [1]: Is a creature that uses Change Shape to become a humanoid affected by spells that only work on humanoids?