How could a group of people defend their fortress against an airship attack?

Question

Introduction

I'm GM, and my 7th-level players are planning to assault a fortress with an airship at night from a distance of roughly 100–2000 feet.

Background

The fortress is an academy for students of necromancy, so I have an excuse for some of its inhabitants to have some highly specialized, unexpected, and non-rules-as-written spells. I imagine most professors will be between level 7-10, while students will either be novices (~lv. 2-3) or adepts (lv. 4-5).

The encounter is supposed to be one of the most difficult encounters they've faced yet, if they decide to do a ground assault, but I don't think they will.

Question

How could the surprised, arcane inhabitants of the fortress defend it against such an assault—without resorting to catapults and ballistas? Most spells have prohibitively short ranges (e.g. Magic Missile is 100' + 10'/level), and the airship will be beyond this range for almost everything.

The only idea I have so far is that the defenders could summon some flying creature(s).

Ideal Answer

An ideal answer would have a brief list of types of spells, abilities and/or weapons that would be effective against an Eberron-type airship flying at a distance of 100-2000 ft, and would be reasonable for a diverse group of 2nd-10th level necromancers to have/use.

To clarify: a comprehensive listing of these things is entirely unreasonable and not in the spirit of this site. I'm looking for summarized lists: e.g. a certain family of spells that typically have long ranges, summoning certain kinds of flying creatures, some bows or crossbows (but only at shorter ranges), etc.

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Further Details

I originally omitted details to make this more broadly applicable for any necromancy fortress getting attacked by an airship. Here is some information requested in the comments about my situation -- and then some! :-)

Fortress description: The fortress is roughly circular and is about 0.3 mi (500 m) in diameter. Thick stone walls surround it to a height of about 40 feet, and a half-dozen guards patrol its walls. There are a number of mostly-stone buildings within the compound.

Most of the party is actually under the fortress in a tunnel, having snuck in earlier in the night. One player was absent during the session, so we half-planned a solo adventure for him. The main party sent this player back to get the airship and its NPC crew. The directions were unclear: this player was either to create a diversion by dropping dozens of thunderstones on the fortress to allow the others to sneak away undetected, or to airlift the players out.

Airship Description: The airship is smaller and faster than typical Eberron airships because it was built for espionage missions. It is about 80' long and has a single, small swivel cannon at the bow. It currently has only 5 NPC crew members plus the player. I haven't determined an exact model.

Setting Description: The stone fortress is situated on a tiny northern island mostly covered in a thick coniferous forest. Pitch-black nights are rare in my interpretation of Eberron because there are several moons visible at any given time. It's an hour before dawn and there are several moons moons visible in various stages of waning. The weather is partly cloudy, and the faintly-golden Ring of Syberis is visible high toward the south.

For stealth, the player tried to smoke-screen the ship as much as possible, though it's difficult to hide the glowing ring of fire that moves the ship.

Answer 1

Since they have casters of levels 4 to 5, they have access to summoning spells, they have access to Fly (3rd level caster), they can fly closer to the airship and fire arrows and spells at the crew.

Spellcasting distance should not be a problem to those with access to Dimension Door (9th level caster) (with a range of at least 680 feet). So the 9th level casters should be able to quickly move around the battlefield.

Attacking at night is subject to concealment rules, even if using siege weaponry. I don't think the attackers have people with darkvision 100+ feet firing those siege weapons. So, even if the target's location and approximate distance is known, they have a 50% miss chance from concealment.

The simplest protection would be spells that works against ranged attacks that does not specifically mention siege weapons, like Protection from Arrows (2nd level caster). Being mostly necromancers, they should have no problem blinding (3rd level caster) those attackers with darkvision, or whoever is operating the siege weapons.

They could always target those aiming the siege weaponry, with damaging spells or force them to fall to their death bellow. Aiming a siege weapon before firing it requires a full-round action, so if you prevent them from using full-round actions, they cannot attack. Spells like Loathsome Veil or Babble (5th level caster) can do that.

A Tornado (if they happen to have a 13th level caster or a scroll) may cause siege combat to become impossible, and winds weaker than that should at least cause a small penalty (-4 for windstorms -8 for Hurricanes). But even a simple rain should cause a -4 penalty on perception checks.

At caster level 9, they have access to Summon Monster V, which includes things like the Babau (darkness and dispel magic), Barbazu (unhealable damage and reach attacks), Large Air Elementals (whirlwind), Kyton (fear effects, reach attacks and possibly can control the airship chains) and Xill (grappling and paralyzing attacks).

The SRD has a few of the necromancer-y type characters from the NPC Codex, which you can use as base for your school of necromancy:

• Blood Bureucrat (CR 2, caster level 3): Has a short range ray of enfeeblement and medium range spectral hand to help deliver chill touch.
• Necromancer Cultist (CR 6, caster level 7): Has enervation, black tentacles, hold person, and could also prepare spectral hand to deliver those ghoul touch spells. Should have access to fear and ray of exhaustion.
• Undead Creator (CR 10, caster level 11): Has some very useful spells, like blink, baleful polymorph, eyebite and waves of fatigue.

Answer 2

Every fortress should have on hand a lyre of building…

While the lyre of building is woefully underdesigned in its second function of actual construction, it's first function is clear:

If the proper chords are struck, a single use of this lyre negates any attacks made against inanimate construction (walls, roof, floor, and so on) within 300 feet. This includes the effects of a horn of blasting, a disintegrate spell, or an attack from a ram or similar siege weapon. The lyre can be used in this way once per day, with the protection lasting for 30 minutes.

I expect that any fortress of necromancers likely used undead labor to construct most of their fortress, but some (ahem) fine tuning may've been done by one or more necromancers with an artistic bent, so the presence of the lyre—even if the fortress is secret, isolated, or both—needn't feel contrived. This GM might go so far as having one creature that is itself an undead creature and capable of playing the lyre having continued to play the lyre nonstop since the fortress's inception, construction on the fortresses vast, sprawling, complicated dungeons having never ceased! (This gives the GM an excuse to have the under-attack fortress rendered invulnerable after a few volleys from the airship: someone finds the lich bard and has him come up top to shred on the lyre to immunize the fortress from attack!)

The range of the lyre of building's defense is limited to 300 ft. from the lyre. With several building inside the compound and assuming smart rather than egotistical necromancers who build gaudy, false buildings to lure potential attackers into telegraphing surprise attacks, one lyre's effect should be enough to for the fortress's population to make its way to the protected building (just like in all the drills) thence to the compound's previously-ever-expanding dungeons.

Note that the lyre's conservative price tag of 13,000 gp puts at least one lyre well within reach of high-level inhabitants. If unfriendlies abound, the fortress could even have several lyres available just in case of such lightning raids by self-righteous, judgmental do-gooders.

…And necromancers in the fortress should have minions and allies

Dan B.'s fine answer is correct: the necromancers' minions and allies are its best bulwark against any assault. Among these, the shadow is likely to prove the easiest to acquire and the most devastating, having a fly speed of 40 ft. with good maneuverability, being incorporeal, possessing a horrifying no-saving-throw-allowed touch attack that creates spawn within 1d4 rounds of those it kills, and possessing an Intelligence score of 6, yet only having 3 Hit Dice. A shadow's Will saving throw of +4 means the necromancers can spam the spell command undead if they must, but it's entirely possible a gang or swarm of shadows may voluntarily assume protection of the fortress with the understanding that the safety of the inhabitants guarantees the shadows' future safety!

Higher-powered necromancers may've forged similar alliances with a gang or pack of wraiths or even a gang or swarm or spectres.

Answer 3

The fireball spell has a long range (400 feet plus 40/level), and it's a popular choice for many spellcasters. I'm not sure how much damage it would do to an airship hull but probably at least some.

Necromancers don't have to summon flying creatures, because they can keep shadows and wraiths and spectres as pets using command undead. This is a really dangerous thing to do, because if one of those creatures goes out of control, it can raise an army of spawns by killing commoners. But maybe these necromancers are crazy enough to do it.

Consider also that some necromancers might be vampires, which means they can switch to a form that can fly directly to the airship.

Answer 4

Even mildly optimized necromancy destroys everyone if given time and materials. For a great primer on how, I cannot recommend K's Necromancy Handbook highly enough.

My original answer follows, but it has a key mistake: I gave bad info about when 5th level spells come online. They are thoroughly in play here, which means your fortress is thoroughly quadratic. I'll discuss some of the options which most stand out to me after the quote.

Capping instructors at level 10 caps available spells at level 4. That's good; 5th level spells are where necromancy goes bonkers (lesser planar binding for nice corpses/haunt-shift shenanigans/magic jar/etc). You still have wizard Animate Dead active, which is good times, as well as Explosive Runes and so even a single level 10 wizard is crazy dangerous.

For example: spend a couple hundred gold worth of onyx zombifying bats, then cast explosive runes all over them. When you need to kill something, you and your bat army fly out to meet them, your bats swarm, and you cast an area dispel magic on them (and fail all your checks). They explode into 6d6 damage per explosive rune, and everything in the area is now chunky salsa (probably including the airship).

That's not a particularly great tactic either, just one which is especially flashy and effective. Good tactics are going to depend heavily on the local resources available. Primarily that means onyx and cadavers, but as K explains "D&D does not currently support an 'available corpses by level' guideline." As such, resources are based on DM fiat, which means tactics are based on DM fiat. You're also going to have to make a number of decisions about how necromancy actually works, which are not otherwise defined (the big one: what do mindless uncontrolled undead do?).

In general though, you probably have a huge number of tiny things patrolling the area. For aerial patrols, this probably means zombie birds and zombie bats. They're cheap; they're effective in large enough numbers; and they're great practice for students. You can get them either directly through Animate Dead (which has a small cost) or through judicious use of Fell Animate and Acid Splash. If you need a real factory, you need something like a bat colony with a spell-stitched badger at the bottom, hanging out and animating two zombats every day without further interaction. The faculty are likely also very eager to get their hands on mohrgs, which also solve the zombie-creation problem.

You also have a zoologists fantasy of undead critters. Just about everything can be animated, and just about everything that comes into the area can be killed. If a hydra comes through, there's a zombie hydra. If a tiger comes through, there's a skellington kitty. If a dragon comes through, etc. Without lesser planar binding, there actually is some limit on what you might expect to see, but still very little. If the college has really had time to get established, they may have breeding programs to keep a ready supply of subjects (e.g. if you can get a breeding pair of cloakers, you have the cloaker aviary supported by spell-stitched skellingtons casting Create Food and Water), though getting the original breeding stock also falls firmly into DM fiat without Lesser Planar Binding.

With more info, I could come up with better answers, but I hope this is enough to get you started.

Fifth level spells are a fun time for necromancer wizards. Entire new vistas open up! My personal touchstones at this point would be Magic Jar and Lesser Planar Binding but it's not like "lesser" spells like Cloudkill aren't at the party as well. Lets discuss:

Lesser Planar Binding is supposed to be a spell where you summon an imp, then browbeat him into cleaning your kitchen for a while (or whatever), then he gets to go home. The thing is, this spell is part of the [Calling] subschool: unlike most summonings an actual imp actually shows up. So if you're a necromancer, you summon an imp, stab him, and then you have an imp corpse to turn into a zombie (decent AC, flight). But the only real limit is the 6HD cap on the spell, so you could do a Dark-Spellwarped-Draconic-Phrenic-Shadow Creature-Half Vampire Fleshraker Dinosaur just as easily (which might make a nice skeleton for the breakfast nook). Whatever fits your idiom, really.

I've had my eye on Kythons (from the Book Of Vile Darkness) as a contender for the most broken chassis, but haven't had a chance to try them yet.

K's handbook has some suggestions on what to look for when deciding whether to make a skeleton or zombie; I won't paste that here but you should read it. Remember, you don't actually need to do much with zombies or skeletons. If uncontrolled undead just stand around or follow their last orders, you can literally stack them in the corner. If you need to isolate them, remember that Wall of Stone came online this level as well: you can trivially build isolated cells for them, then pave over the top and use it as a basketball court. To break into your stash, you'll need a serious burrowing creature. Probably that means a zombie umber hulk standing awkwardly in a display case labelled "break glass in case of emergency".

Remember that there are clerics about, so every undead is going to be created inside a Desecrate spell (with altar), so they will all have +2 hp per HD.

Magic Jar lets you replace your feeble nerd body with an awesome one. You can use Lesser Planar Binding to find whatever one you like, or you can just hop into whatever zombie dragon you were keeping around anyway. This has a couple quirks to be aware of with respect to range between your original body, the focus, and your current body. It only check distances when someone dies or when the duration expires though, so it's very workable. Probably you want to do something like put the focus in a bird zombie, then carry the bird with your possessed body. If you die, the bird flies back. You can possess the bird if you need to. If your opponents were careless, you even get to try to possess them (leaving your demon/zombie to rampage in the meantime).

With a whole college of necromancers, you get even stupider options. For example: stick a half dozen focii and a collection of zombies inside a hollowed-out draco-zombie, and you have a mothership that rains down full-casting zombie succubi (again, adjust to suit your idiom). Every time one dies, he just grabs another body out of the ship and comes right back. Is the draco-zombie mothership actually good? No idea, but it's a great example of the crazy-go-nuts options which open up to necromancers at this level. It doesn't take any DM fiat either, this is all world-agnostic* combinations of core material.

(*okay, not entirely world-agnostic. If succubi simply don't exist in your multiverse, they can't be called and made into zombies. But anything that exists anywhere in your universe is fair game, and it's usually fair to assume that means anything in the core books.)

If you were playing 3.5, I would add Haunt Shift to this list. You're not, so I won't, but it did add a whole new layer of crazy.

Answer 5

You can't hit what you can't see...

If the fortress is supposed to be hidden from the world, the occupants may have various fog and darkness spells prepared. You can't zombify too many passersby before a large force comes looking after all...

I saw the standard Fog Cloud and Solid Fog spells are rather small in area of effect, but an extended version covering a decent area of the fortress will make aerial assault almost useless. Any shot into the fog results in a solid thud as the projectile hits earth or stone.

If the airship descends into the fog, it may crash into tower as well exposing itself to all the Necromantic fun the defenders have in store.