In previous editions, certain monsters had the ability to cast gate. For example:
- Titans (now Empyreans) could cast the spell innately, as a spell-like ability
- Epic prismatic dragons were assumed to know the spell as a sorcerer
- Solars could cast as 20th level clerics (so, if prepared, could cast gate) and in any case, could cast wish as an innate spell
- Pit fiends could wish once per year
I’m certain I missed a few. The Solar, in particular, was an amazing source of the so-called gate cascade, because of course the gate spell can bring in a known extraplanar creature. If you summon one solar, within 24 hours you will summon every solar, since they can call each other using wish and using clerical prepared slots.
Terrifying to every would-be necromancer and diabolist; terrifying to every paladin and high priest. Presumably the Heavenly Choir is kept from doing this because the other angels are busy, but it’s still a sobering thought that all resources that can be freed can be transported instantaneously anywhere, if even one of them is brought there first. Why would the first balor on the scene not wish in an infinite number of reinforcements? Anyone capable of this scheme can just spend weeks building up their forces, even if they’re not as exponential as the solar.
In 5e, it’s fixed in a few ways. No monster in the MM has planar ally or gate as an innate ability; nor do any casters have it explicitly listed as prepared. As a result, even the sorts of beings previously capable of participating in such a cascade can’t do so effectively. Cutting this mechanism to the bone are the widespread removal of spell-like abilities and spellcaster-equivalent abilities; while nagas and sphinxes still cast, they’re monstrosities (and so not summonable); a full hag coven is fey or fiendish, but cast from the wizard list where they can’t bring more of themselves in, and so forth. In general, creatures that couuld gate got downgraded to plane shift, which I’m in favor of.
But we still have vestiges of this self-increasing extraplanar influence around the edges. The most obvious: Most fiends have a sidebar ability which can, with some probability summon some sort of reinforcements. Those abilities don’t make a ton of sense to me as a combat ability (which is what they’re for: short duration, denying the summon-ee their own instance of the ability), but do work well as inspiration for a story ability: summon a spirit of disease (y’know, a type I demon) in an old house, better expect an infestation of minor imps along for the ride.
So long story short: how do summonings work? It’s easy to get an in-combat helper with a variety of conjure elemental or conjure celestial or what-have-you effects. The unearthed arcana old black magic article provides a few more wizardly options (The concept is straightforward: call a fiend with challenge equal to spell level; it challenges your authority each round until it breaks free). Want to have it stick around longer? Cast an inverted magic circle, summon the creature on-target, make sure it’s one of the critters that can’t get out on its own (maybe a dimensional anchor?), apply a planar binding and you’re golden, assuming it fails all the saves. One in-combat helper transformed into an out of combat helper, and all it took was a 3rd level spell, a 5th level spell, maybe a 4th level spell, and the actual conjuration which might require some minor risk to life and limb. Gate is still around if you’re a PC, and is even better: it lets you go fishing for creatures of arbitrary power, albeit at very high level.
In some ways, then, the cleric’s planar ally feels unfair. Poof, one ally of arbitrary strength, in exchange for wealth appropriate to service; no haggling, no risking your face getting melted. There’s probably some lawful good deity of goodness; you’re probably their priest, probably engaged in some save-the-world mission; surely we can come to some wealth-by-level-appropriate arrangement.
But that demon cult down the street is probably after just such an arrangement as well. If they can get one caster capable of 6th level spells, that caster can planar ally, and I presume any Unwholesome Power interested in sponsoring the local chapter of the Demogorgon Fan Club has some auto-extracting self-installers on call for just such an opportunity.
So why hasn’t the world ended yet? It’s basically got to be because this doesn’t work for some reason: because there is no such autoextracting self installer possible, or no power interested in sponsoring it, or similar. Or alternatively, the world has ended, this does work, and the big problem is that we’re in one of those periods between apocalypses while things rebuild.
Let’s assume for the nonce that it’s because summoning creatures on the higher end is rough on reality, and reality fights back: you can call up a pit fiend, but if you haven’t put the work in to roll out the welcome mat, calling up the next few unwholesome allies gets much harder, and only manages a thin shower of lemures or, at least, PC-appropriate foes like spined devils. What you wanna do here is build up a huge background of fright and pain and death for weeks and weeks. Then, when the ‘fiend shows up, there’s enough background horror that you can keep calling them up, and get a proper apocalypse going. We could probably build a big table of how much pain to earn this many pit fiends but it doesn’t seem worth it to me; it moves at the speed of plot.
What doesn’t move at the speed of plot is arcane background radiation. Here, have some “hanging around waiting for the dead to be raised, the puzzle box to be opened, the wrong incantation to be flubbed” random encounter tables.
These tables are organized by tier; I suggest something like rolling a d8; on a 1, use the single next table up; on a 2-5, use the tier appropriate to the site, and on a 6-8 use that many instances of the table of the next tier down.
If you’re not sure which table to use, I suggest a d6: 1-3 fiend, 4-5 undead, 6 other.
5: spined devil
6-7: hell hound
9-11: shadow demon
1: barbed devil
3: bone devil
4-5: chain devil
8-9: hezrou (abyssal ghast)
10: night hag
note: unique individuals like cambions aren’t random encounters; yugoloth and servitor demons are rarely randomly encountered.
1-4: horned devil
10: ice devil
9-12: pit fiend
Always a shadow.
5-6: Poltergeist Specter
Always a wraith
Always a Dread Wraith (per Scot Metzger’s Monster Expansion)
Always an avatar of Death (per the Deck of Many Things) with 200 hit points, additional damage immunity to mundane weapons and elemental energies, its reaping scythe damage replaced with an even 50 force and 50 necrotic damage to all foes within 5 feet (as before, no to-hit necessary; maximum hit points are automatically reduced by the necrotic damage), and etherealness as an action.
1: gas spore
2: gray ooze
3-4: rust monster
5-6: swarm of ravens
7-8: swarm of insects
9: giant centipede
10: giant wolf spider
11: slaad tadpole
1-3: gibbering mouther
4: intellect devourer
5: death dog
9: phase spider
10-11: ochre jelly
12: black pudding
1-3: red slaad
4-5: blue slaad
6: invisible stalker
8-9: green slaad
10: grey slaad
11: death slaad
9-12: Shoggoth (from Metzger’s Epic Monster Expansion)
1-6:Mi-Go (from Metzger’s Epic Monster Expansion)
7-10: Star-spawn (from Metzger’s Epic Monster Expansion)
11-12: Elder Thing (from Metzger’s Epic Monster Expansion)