To keep it simple I wanted to roll as few dice as I can. In this case if you follow the steps below you can determine the level, race, sex, weapons, armor and stats by rolling very few actual dice. Steps Four and Five are optional.
For fast results choose a level range, then roll percentile dice for race/sex and roll 1d8 for weapons/armor. That is it. Do steps 4 and 5 if you need extra details.
STEP ONE - DETERMINE LEVEL
If it is a single assassin that is encountering a PC, roll 1d4 and add the result to the level of the PC being encountered. eg. A level 5 character should be encountering an assassin of sufficient level to provide a challenge, levels 6 to 9.
If it is multiple assassins that the PC or party of PCs are encountering, determine how many assassins they are encountering (possibly a whole assassin's guild, which is 7d4 assassins + 1 level 14 or 15 assassin guildmaster) and then set the average level of the assassins to be roughly 4 levels below the level of the PC. Thus a 5th level group of PCs should be encountering a group of assassins which are mostly level 1s. eg. Eight level 1s, four level 2s, two level 3s, and one level 4.
For higher level PCs the system I like to use is to subtract 7 from the PCs level, and then roll a d6 and add it.
For example if the PC or PCs are 15th level, you roll 1d6+8, resulting in assassins which are levels 9 to 14. A large group of assassins of those levels should provide sufficient challenge for a 15th level character or characters.
If using a smaller group of assassins, comparable in number to the number of PCs, then subtracting 3 and adding 1d6 should create a randomly leveled number of assassins for the party to deal with.
Once I have the level of the assassin I also use this to determine the bonuses to any magical weapons and/or armor being used by the assassin. See Step Three further below.
Level Bonus
1 to 3 --
4 to 6 +1
7 to 9 +2
10 to 11 +3
12 to 13 +4
14 to 15 +5
Notes - Maximum assassin level is 15 and level 15 assassins are usually solitary figures / leaders of their own guilds. If playing with racial level limits, assume that all high level assassins are automatically human (or possibly half-orc if you allow those in your game) and skip Step Two below.
Roll percentile dice (1d100) and check the result below:
01 to 29 Male Human Assassin
30 to 59 Female Human Assassin
60 to 63 Male Half-Orc Assassin
64 to 67 Female Half-Orc Assassin
68 to 71 Male Half-Elf Assassin
72 to 75 Female Half-Elf Assassin
76 to 79 Male Elf Assassin
80 to 83 Female Elf Assassin
84 to 88 Male Dwarf Assassin
89 to 92 Female Dwarf Assassin
93 to 96 Male Gnome Assassin
97 to 100 Female Gnome Assassin
STEP THREE - WEAPONS and ARMOR
Roll 1d8 and check the result below:
1 Leather Armor + Longsword + Longbow (with 20 arrows)
2 Leather Armor + Battle Axe + 2 Throwing Axes
3 Leather Armor + Morningstar + 2 Javelins
4 Leather Armor + Scimitar + 2 Throwing Daggers
5 Padded Armor + Shortsword + Sling (with 20 sling bullets)
6 Padded Armor + Spear + Heavy Crossbow (with 20 bolts)
7 No Armor + Scimitar + Curved Dagger
8 No Armor + Shortsword + Dirk or Dagger
I follow the assumption that one or both of these weapons is magical if the assassin is 4th level or higher. 1 to 40% the main weapon, 41 to 80% the missile weapon/backup weapon, 81 to 100% both weapons are magical. If both weapons are magical, assume that one of the weapons is 1 or 2 points less magical than the other. So for example if a 15th level assassin has a +5 Longsword, then he also has a +3 or +4 Longbow.
2nd Note - You are not done calculating the AC of your assassin(s). To do that you will need the armor, the bonuses on the armor (as determined by level), plus the Dexterity of the individual assassin.
3rd Note - You should give extra attention to designing the weapons and armor of a guildmaster. They should have multiple magical items, possibly some invisibility and/or healing potions or something unique like a strange sword named "Blooddrinker", etc. Spice it up a bit!
STEP FOUR - ABILITY SCORES
When designing PCs normally you roll ability score stats first, but rolling stats for assassins isn't always necessary. For saving extra time you could simply assume the NPC assassin has the following stats:
Str 12, Int 11, Wis 11, Dex 12, Con 11, Chr 11, Com 11.
But if you want more details for more "elite assassins", I recommend the following two options.
For Non-Guildmasters roll the following:
Strength 1d6+12
Intelligence 1d6+12
Wisdom 11
Dexterity 1d6+12
Constitution 11
Charisma 11
Comeliness 11
For Guildmasters roll the following:
Strength 1d4+14
Intelligence 1d4+14
Wisdom 2d6+6
Dexterity 1d4+14
Constitution 2d6+6
Charisma 2d6+6
Comeliness 3d6
Note - Now that you have the stats that you can finally calculate the assassin's Armor Class using the Dexterity modifier (Dex 15 / -1, Dex 16 / -2, Dex 17 / -3, Dex 18 or 19 / -4), the armor type (-2 for leather, -1 for padded, and apply the negative if the armor is also magical.
eg. Base AC 10, -4 for Dex 18, -2 for Leather Armor, -3 for the assassin being level 10 and having +3 armor. Total = AC 1.
STEP FIVE - DETAILS, ONLY IF NECESSARY...
Extra details like name, age, weight, height, etc. Make it up on the fly, and only if you actually need to. Often it isn't necessary for characters to know every detail.
Also best case scenario is the party leaves the dead assassin(s) in an alleyway and never bothers to loot their bodies and thus don't know that they might have magical loot. If you are like me and like your PCs to not be collecting huge piles of magical items all of the time I recommend the following: Say the armor got ruined when the PC killed them, and the enchantment on the magical armor immediately faded. The PCs will never know it was magical. They might figure that out via metagaming and math, but when they check the armor with Detect Magic you can just say "There appears to be a residual magical energy on the broken armor. If it was magical once it now appears to be broken."
Alternatively the assassin could have also have died spectacularly by falling into a waterfalls, pit of lava, eaten by crocodiles after falling into a river. Their magical items conveniently disappear along with their corpse.
The guildmasters are the characters that should have the most details. PC contacts (Spies? Allies? Double agents?) within the assassin's guild should also be of special note and receive special attention. If the guildmaster was also a long term villain, then looting the body of their felled opponent should bring a special kind of glee as details are revealed about the assassin's past/future dealings, magical items, interesting scars, tattoos, letters of writ, etc are found on his person..
