Thanks all. That more or less settles my worries for the authorities, so to speak.
But what about the opposition? Surely they can't all be grand master assassins, high priests and elite warriors. How to do this without it being a slog because only low level enemies, or far stretched because apparently the best of the best all happen to be members of this cult?
Mob Rules from the DMG.
Not only will combat speed up significantly, but the automatic hits will make the squishier members of the party feel the pressure.
For example, lets take the CR 1/2 Thug!
For starters, they get two attacks per round (with clubs), and Pack Tactics, which is amazing for Mobs. Unfortunately, we are going to start out using their crossbow attack.
Lets generously assume that one of the back row [-]targets[/-]PCs has an AC of 16.
This means one of our thugs needs a 14 to hit them normally .
But wait, because we are using a passive check instead of a roll due to the Mob Rules, we can use those rules to our
advantage as well. Using Advantage as a +5 bonus, a la passive checks, means they would only need a 9 to hit provided they can leverage Pack Tactics.
So using the mob rules, this means that one out of every two attacks launched by our mob of thugs will automatically hit the PC. Even if the PC manages to take up half-cover behind one of their party members! Using the average damage of 5 per hit landed, a mob of 15 thugs will deal 35 damage per round (or 40 damage on the second round).
Now, It's worth noting that mobs can be tweaked and still keep the same CR, just so long as they don't up their optimal DPR. This means our Thugs will take a page from the PCs, and be utterly scummy crossbow experts that use hand crossbows to get two attacks per round at range, at a +4 attack bonus (because experts!) for 1d6+2 damage (again expert bonus, so as not to change any other stats) each.
Against the same 16 AC, they now only need a 12 ( a Pity we couldn't get it two points lower), and don't even need Pack Tactics to land one out of every two attacks thanks to the Mob Rules (However, using Pack Tactics lets this work against targets that have 18 and half-cover, or 21 ac without cover!) Also, they are all doing two attacks, which means one of their attacks hits per round. With 10 of these crossbow thugs, that poor PC is looking at eating 55 damage to start with. And 15 crossbow thugs deal an absolutely deadly 82 points of damage per round.