I know, I know. Assassination charts aren't exceedingly popular. But, let's go through what ACKS already gives the assassin. In fact, there's an assassination table hidden in the system!
First, let's look at Blackmoor's Assassin. For clarity of purpose, I'm capping levels at 14, and separating out the columns into individual levels.
Let's see what 1E does for us:
ACKS's Assassin is a d6 fighter, essentially, that can sneak about. The important part about that is the Damage Bonus ACKS supplies to it's classes - the "Fighter" classes in particular. That ties into ACKS's succinct Cleave system that replaces the old system of multiple attacks vs low HD opponents.
The Assassin is getting that Damage Bonus, and Backstab (backstab being a prerequisite for assassination attempts anyway). How often are we likely to outright slay our target with just the Assassin as is?
A statistician would probably be able to toss out some sort of equation that would let us know. I'm not one, so, I'll brute force it instead. I'm a man of the times, and I let computers think for me. That, and Stats wasn't my best of classes in college.
I've taken a single Assassin, and, for each level from 1 to 14, I've pitted him against 10 million opponents, for each level from 1st to 14th (140 million opponents per level, 1,960 million NPCs attacked in total). He's backstabbed them all, and we've counted how many he's dropped in one hit. (we assume he always hits).
This was using a d6 weapon, with no bonus from Strength or magic. As I later found, only the d6 roll is multiplied, the damage bonus is static.
And that will tell us our "natural" assassination table for ACKS. Voila:
My color coding there is from an earlier model, in which I'd simply pitted average and maximum backstabs per level versus average and maximum HP per target level.
I rounded down always, unless I could round up to 1%, to make the table look better.
So there's that. It's nowhere near the original source material. On a lark, let's allow the Assassin to double his damage bonus during backstabs:
Not much change, as expected.
I've got a better idea though. What assassin wouldn't use poison during an assassination? The one that's bad at her job, that's who. You're already murdering someone, they're not going to hang you any harder for using poison if they catch you.
Here, then, is an ACKS Assassination Table, presuming the assassin is using poison. This is the chance the target will die based on (first) damage, from the natural ACKS table above, and if not, failing the poison save. I've turned the save around from something the target (DM) rolls to a percentage chance on the players part. I'm using Fighter saves in this table, as it's worst-case-scenario, in general. A 14th level Fighter saves vs Poison & Death on a 5+, so that's a fail on a 1->4 on a d20, or, 25%.
It looks a bit more like the Blackmoor table.
For comparison, here's a table of how the Assassination Hijink from a Hideout goes. Note there's a -1 penalty per level the assassin is lower than the victim, but not the other way around, by the book.
Note a roll of "1" for the 14th level assassin, while a success, also means they've been caught.
This last table is what a "natural" assassination table looks like for a Thief, who doesn't enjoy a Damage Bonus. I don't recall what the backstab progession was in Blackmoor or 1E, but you can sort of see how much of advantage was given to the Assassin comparing this with the tables at the top.
Anything useful here? Not really. If a player was having an adventure which culminated in an assassination, I might let him roll against the "Base ACKS With Poison" table above, just since it might feel more dramatic than a low damage roll and then hoping for the DM to fail the save vs poison.
Which reminds me I should throw down a players-roll-all-the-dice conversion, just because I'd always wanted to do it.