In my experience, infiltration types of missions are not difficult to prep for as long as all you do is what the NPCs themselves actually have to do. That is to say, set up rough schedules of what the guards will be doing, what countermeasures the NPCs will have, and what the numbers of reserves look like and what happens when the alarm is sounded. Then you treat the entire mission as one block of XP.
Thanks Bacon Bits.
Well, the hard part
is prepping what the NPCs do in sufficient detail and verisimilitude! At least, that's my experience. I'm definitely planning to do a rough camp schedule, table of possible complications, table of overheard snippets of conversation, what happens when the alarm is triggered, total camp forces, and try to address some magical countermeasures to scry+teleport (these are the hardest for me).
Simply put, trying to force encounters to go a certain way with stealth and infiltration will just often not work because the my players almost never do things the way that you think they will. They will come up with multiple ideas that you never even considered so you won't have any plans to address. One of them will have disguise or illusion magic and walk right past your guards. Or they'll use charm magic to make friends out of enemies long enough to get what they want. Or they'll split up with one party causing a distraction while the others get what they're looking for. Or they'll have a timed distraction. Or they'll make a coordinated strike and hit fast and hard.
Oh! Maybe I wasn't clear? I certainly wasn't trying to force them to go a certain direction. The assassination plan was embraced by the players themselves. At the end of our last session they were presented with a couple options: (1) Assassinate the Lizard Emperor at his forwarding base camp where his defenses will be less extensive than in his capital city, (2) Stop a ritual by the Lizard Emperor's warlock emissary attempting to summon a demon lord, (3) Form an alliance with smaller lizardfolk tribes and wage war against the Lizard Emperor's Akabkan confederacy, and (4) Try to sow discord within the Akabkan confederacy to get the various evil lizardfolk power groups fighting amongst themselves. They discussed and choose #1.
Also, the sheer opposition of the camp is far in excess of what the party can handle – I telegraphed this through allied NPCs sharing some info to the players, and because I have a pretty good handle on their upper limits based on past encounters. This seems to have dissuaded them from a head-on assault. For your reference, once the PCs bypass the three concentric rings of defense, the camp (if encountered all at once) probably amounts to 120,000 adjusted XP. For reference, my party
with their NPC allies have an adventuring day XP budget of around 50,000.
Don't think about what the party can do, per se. Think about what kinds of things the NPCs would be interested or could feasibly stop. Sure, the PCs can teleport and use scrying. Do the NPCs actually have secrets that could be scryed? Do they have other enemies that are known to teleport? How paranoid are the NPCs What kinds of threats do they already routinely face? What simple things can they do? Do they have perpetual light spells? Alarm spells? Arcane lock? Glyph of warding? Guards and wards? Symbol? Forbiddance? Note that many aburation spells are rituals. What kind of escape plan do the NPC leaders have? What do they care about the most? Who do the NPCs know that would sell or give them resources?
Do the NPCs actually have secrets that could be scryed? Yes, absolutely.
Do they have other enemies that are known to teleport? No.
How paranoid are the NPCs? Very. The Lizard Emperor has schemes within schemes, and his forward camp is at the fringes of his territory, a few miles from a sacred site revered by multiple lizardfolk tribes.
What kinds of threats do they already routinely face? The general territory – the Valley of Dread – has numerous threats like goblins, pterafolk, spirit nagas, undead, dinosaurs, etc.
Do they have perpetual light spells? Alarm spells? Arcane lock? Glyph of warding? Guards and wards? Symbol? Forbiddance? Potentially
alarm, but none of the others. The warlock emissary is not present (he's back in the city). The night hag disguised as a lizardfolk mage "Nataxl" is an innate caster with at-will
detect magic &
magic missile, and 2/day each:
plane shift (self only), ray of enfeeblement, &
sleep. Being a hag, I can give her "weird magic" that suits her theme. Also, the Lizard Emperor can innately cast 1/day each:
darkness, pass without trace, &
silence. And his two sub-chiefs – who I'm still writing up stats for – might have 8th level ranger spellcasting, meaning up to 2nd level ranger spells, of which
alarm would qualify.
What kind of escape plan do the NPC leaders have? The Lizard Emperor can become invisible in dim light and darkness, so he would use that to escape until he could reach one of the few quetzalcoatlus mounts/out-fliers. The night hag simply will cast
planeshift or use Etherealness to escape...though is a way for the PCs to convince her not to flee (if they're not trying to kill her).
What do they care about the most? The Lizard Emperor wishes to unify/conquer all lizardfolk tribes, destroy any rival lizard kings, and then conquer the land to take it away from humans/goblins/undead and establish lizardfolk on top. The night hag is trying to recover a magical shadow of the god Semuanya that escaped her magical binding; she is concerned this failure will inspire derision from her coven sisters, or possibly punishment by her master Acererak. They are camped by the sacred site – Semuanya's Birthing Stones – to make sure the fractured god Semuanya is not restored, and also hoping to incite local tribes to being first aggressors in order to justify seizing the sacred site.
Who do the NPCs know that would sell or give them resources? The Lizard Emperor and his forces are already well-supplied by the Akabkan lizardfolk confederacy & their capital city. The night hag may have traded with her coven sisters for weird magics.
This kind of prep is when the DM gets to roleplay. You get to think like the NPCs and make plans and formulate them. Use your resources as best you can, and don't pull punches if the PCs walk into your preparations (but do have potential backups in mind if the PCs suddenly can't succeed). IMO, don't think so much about challenging the players as much as you are thinking about playing the NPCs correctly. You can always adjust the latter to fit what the former requires.
As you can see, there's a lot of moving parts. One of my challenges is distilling all this into actionable usable things at the table that support the player's desire for a stealth-focused assassination mission.