My players have attracted the attention of powerful individuals who want them... removed. I'd like to have them attacked by some assassins; I think the encounter will be a very vivid way to drive home the point that the quest they're on is bigger/more complicated than they realized at first.

This is a large party - eight characters at level 4 - and so I've been having a bit of trouble scaling encounters appropriately because even Xanathar's expanded content on building combat encounters caps out at a party size of six. So far the issue has been too easy of encounters, but I'm worried that this next encounter will swing too far in the other direction, and I'd like some advice.

The Assassin in the basic rules is CR 8, which seems like it would be tough but beatable for an eight-player party. But I'd like to have two, maybe three assassins go after them, which has the potential to really mess the party up with the Sneak Attack ability that assassins have. 

More experienced DMs: do you think two CR 8 assassins would be too much for eight level 4 PCs to handle? Should I debuff them by either removing one of their special abilities, lowering their HP, both, something else?

I'm no expert, but I've given situations like this a lot of thought.

I have found it useful to think about your encounter in terms of how many attacks the party would get per round versus how many attacks the enemy would get. I have a party of five level four players that consistently fight considerably above their CR rating because of this. Instead of two CR8 assassins, I think they might find 10 assassins of a mix of CR1, 2, 4 (six henchmen, two lieutenants, and a leader) more of a challenge, especially if they start off with ranged attacks. Admittedly, that's a lot going on, but throwing a big group of monsters at them allows you to play to the NPCs strengths and it also guarantees that a great roll by them or a terrible roll by you doesn't swing the entire encounter.

If you do like the idea of one or two big bads going after a party of eight, I would suggest using hit and run tactics to sow confusion and draw the players out of a traditional skirmish. Don't let them mob you or you will 100% go down quick due to the sheer number of incoming attacks from the players. Use a good-sized battlefield and move around it a lot. Maybe give the assassin the ability to cast Misty Step at will similar to how the Hobgoblin Iron Shadow does. Alternately, Fog Cloud is great for breaking line of sight and letting the assassin slip back into the shadows. Ranged attacks are essential for singling out users of concentration spells.

Also, with a group that size, I would suggest not ever having any combat encounters that are not relevant to the story. I picked that one up from Colville.

You could easily have the assassins target specific members of the party. When that character goes down, have the assassins withdraw with a warning.

If the party persists, hit them at home. Not their actual home, but have their favorite bartender or something attacked or killed.

TexasDevin - Thanks for the feedback! You make a great point; a group of adversaries with varying initiatives would probably be more challenging. I was hoping I could get away with less prep work for this upcoming session, but alas that will not be the case. ;) I'll do some more digging to see if there are good stat blocks I could use with a slight reskin. Also, I agree with you 100% regarding having only plot-relevant combat encounters - I don't do random encounter tables at all. Fortunately, my players are also good about avoiding unnecessary combat; they are the opposite of murderhobos, which is a pleasant surprise.

Wysperra - Those are both excellent ideas, thanks! I think I may incorporate that second one; they have several NPCs that they are close to, and targeting one of them would definitely shake the party up.

I'd suggesting using a calculator like this one to scale encounters for your larger-than-average party. The XP thresholds for your party are as follows.

• Easy = 1,000 XP
• Medium = 2,000 XP
• Hard = 3,000 XP
• Deadly = 4,000 XP

If this is going to be one of the only encounters for the day, then you can go for something that exceeds the Deadly threshold by a little bit. I agree with TexasDevin's advice to use a larger group of assassins, but the specific suggestion he gave (six CR 1, two CR 2, one CR 4) is pretty deadly at 6,400 XP.

The standard Thug and Spy stat blocks (CR 1/2 and CR 1) might make for good henchmen. Also, you can always lower the assassin's CR by altering the HP, AC, and damage per round. The assassin's damage is the main driver behind its high CR.

Thanks, jreggers! I use milestones for leveling so using XP to calculate encounter difficulties didn't even occur to me.

I would actually do 3 assassins. 

Remember you can always play with their stats to balance them a bit. Lower hp, lower ac, take a few points toward the + to hit. I would have one or two assassins brazenly just attack them, thus distracting them from the sneak of the other(s) assassins, it will put a little twist on the encounter for the party, will make it feel organic, be challenging while giving the party a fighting chance home AND helping you get the point across that something deeper is going on. 

I used the old "assassins in the night" routine on my party earlier in my campaign. If you need a little extra push to indicate this was a hit instead of a random robbery, you can have each assassin have a piece of paper on their person with a sketch of the whole party. So if any of the assassins are killed, even if one or two get away, the party will search the corpse and find the evidence. If you need to really beat them over the head with the obvious, make it a WANTED poster :p

It's also nice because if the hit fails, the players can be pretty sure a bigger and badder hit job will follow eventually.

An update on this topic: in the end, I had the assassins attack during a masquerade party, which meant that the PCs had to deal with protecting noncombatants in addition to saving their own skins. I used one CR 8 Assassin, two CR 3 Archers who used flintlock rifles instead of bows for higher damage, and three CR 1 Spy(ies) who used daggers instead of short swords because it made more sense in context (easier to hide in their clothing). I broke up their initiatives by CR, which meant that there were three enemy initiatives.

The party was genuinely challenged! Especially because they actually cared whether the non-combatants were killed, and because it was at a masquerade they did not have their armor on (the fighter in particular was challenged by this, as his AC was only a 9). However, they managed to make it out with no fatalities, partially because I accidentally lowballed the Assassin's sneak attack damage. Oh well. They could definitely tell the difference between this encounter and earlier encounters, regardless, and they were impressed. Thanks to everyone for their help!