Homebrew: Arcane Cannon (Eldritch Machine)

I don't know if something like this already exists in official Eberron lore but I hadn't heard of it so I thought I'd give this a try.

Arcane Cannon
Large Eldritch machine, uncommon (requires attunement by a spellcaster)
Armor Class: 15
Hit Points: 75
Damage Immunities: poison, psychic

The arcane cannon functions as a giant wand (arcane focus) allowing anyone who can use a wand as an arcane focus to attune to it. Once attuned, a spellcaster can use the arcane cannon to cast spells as if the cannon were any other arcane foci. To cast a spell through the arcane cannon the spell must have a range other than touch or self and must be able to do damage. Any spell cast using the arcane cannon as a foci has its range increased by a factor of 10 and all of its damage doubled. Thus a fireball cast as a 3rd level spell through the arcane cannon would have a range of 1500 feet and do 8d6 x2 fire damage while an Eldritch Blast spell cast through the cannon would have a range of 1200 feet and would do 1d10 x 2 force damage.

The size and weight of a heavy cannon, this eldritch machine is still too large and bulky to be wielded in most dungeons environments and is instead meant to operate as a very powerful siege weapon.

I don't know the balance on this. On the whole it might be a little weak compared to siege weapons, but I didn't want to go too far overboard. Ideally I think Eberron just needs bigger weapons and I like the idea of a way to make war magic an actual thing that while it exists is unlikely to have an impact on most campaigns. I've also thought about having a divine version of this or just making it general enough to be used by any spell caster. I even thought about not limiting it to damaging spells (so a cleric can use this to cast a mass heal wounds that would affect an entire army).

Thoughts?

Keith Baker has mentioned on his website and in his books something called a siege staff or siege wand. Basically a tree sized wand/staff. I would guess most would be enchanted to produce a specific spell effect, such as a super-sized fireball. But it wouldn't be a stretch to assume ones that generally amplified spells. Personally, I would adjust it to rare, and require special training, maybe via a feat, for a spellcaster to use it without it blowing up in their face.

While a good thing for NPCs to have, imagine if a player gets a hold of this. Requiring a feat to use it will at least ensure that your players couldn't use/abuse it right away without some kind of investment.

A siege staff. That's exactly what I was thinking. I thought maybe Eberron had that idea floating around someplace. Cool. Thanks for that.

As for training, well the idea for me is to introduce a way to allow war magic (that is magic that can affect an entire battle) without limiting it to only high-level spellcasters and without giving players anything they could normally bring with them in the campaign. That's why it's a large, heavy cannon-sized object. No way they're bringing that with them anywhere but a battlefield. If I also force them to take a feat that seems like too much. I don't want them to have to invest in something that's only going to come up once or twice in the campaign.

It did occur to me though that maybe it should be limited someway. Maybe the base cannon is an uncommon magic item but you can't cast spells through it higher than 3rd level, while the rare version can handle spells up to 5th, the very rare up to 7th and the legendary up to 9th. Or maybe the base version is common but you can only cast cantrips through it but it goes up from there.

I've also toyed with the idea of it increasing the area of affect of spells or even adding area of effect to single target spells. Imagine a war warlock being able to cast an Eldritch Blast through this thing that scours the battlefield. Maybe too much but I like it.

Ultimately though I'd like to modify this to be used in a Spelljammer type game. Imagine a ship piloted by a wizard with one of these. Now imagine a squadron of them. We could finally bring some real Star Wars/Guardians of the Galaxy style space battles to Spelljammer and maybe get away from the Age of Sail stuff (not that Age of Sail isn't great too).

I disagree. As an analogy, if you have a person that is trained to drive a car and use a gun, without any training could they operate a tank? Similarly, I don't see any magic user being able to use a siege wand without specific and possibly extensive training. From my perspective, a siege weapon is much more sophisticated than a normal wand, and doesn't simply enhance magic cast or focused through it. Rather, I see them as specially enchanted and containing so much magical energy that it could potentially catastrophically explode if used wrong or it the wards and sigils containing the magic are damaged enough. I believe that while siege wands would have been used with regularity in the Last War, they also had that trade off of potential destruction. Thus they would have been used only from a good distance with sufficient protection. Much like how artillery shells must be carefully stored and protected lest they become a liability as much as a weapon. That's why I personally don't see them being able to cast any spell poured into them, but only specific magical uses determined at their creation. And that is why I don't see just any wizard or artificer being able to use them.