But during this past weekend's session, Vivuli turned himself invisible and snuck up on a magma elemental completely silently. I fully expected him to assassinate the unssupecting magma being and, having myself forgotten that house rule in the three weeks since we last played, was quite surprised when Viv rolled a bunch of damage dice instead. I tried to revoke the house rule then an there, though a cooler head -- Spawn of Endra's -- wisely advised me not to make a rules change right then in the heat of a melee battle. I concurred.
Yet looking at both assassination options mathematically/probability wise -- NOT my strong suit -- reveals that the difference in likelihood of each of these two approaches resuting in an instant kill may actually be minimal. This is particularly true in this campaign, in which the players are granted once-nightly d30 rolls. If Viv were to use his d30 as an attack roll in an "assassination attempt" situation, this gives him a 33% chance of achieving a critical hit (20 or better), so combining that with the "extra damage dice" house rule could allow Vivuli to achieve an "instant kill" in most cases simply by dint of the amount of damage he inflicts, i.e., (weapon damage + 5d6) x 2.
I asked my good friend and very knowledgable D&D rules expert Carl to jot down his ideas on this matter:
The assassin as written is basically a thief sub-class specializing in the backstab ability of all thieves, disguise, and poison. To understand the assassin's "Assassination" ability we have to include the regular backstab, because the first prerequisite of the assassination is that the assassin must first attempt and succeed at a regular backstab attempt (AEC p.10). The assassin also has to achieve surprise on the victim. If those two conditions are met, the assassin has a base 50% chance to kill outright a victim of equal level or monster HD to the assassin. For each level difference between the assassin and the target, the chance of success is altered by 5%.
What happens on a failure? The assassin still stabs the victim in the back, doing regular backstab damage, but nothing else extra happens with a failed assassination attempt.
How much damage does Vivuli do with a regular backstab? A successful backstab multiplies all damage by two. Viv typically uses a new steel handaxe for close quarter combat, so his base backstab damage is 2(1d6+1d3). His backstab averages 11 damage then.
Now let us look at the houseruled Assassination class feature, which gives a bonus of +1d6 damage per level of the Assassin to the attack instead of granting a percentage chance to kill the target instantly. Obviously in this case, if the damage of the attack is equal to the target's HP, the end result is an outright assassination. But a result that does not kill the target outright is not really a "failure" in the sense of the term that the assassination attempt could fail as originally written. Let's look at the numbers.
Vivuli is Assassin level 5. Lets compare the numbers at equal level first. A 5 HD monster has 22.5 HP on average. Viv's assassination does 23 HP on average - he rolls 6d6 and a d3 (his base 1d6+1d3 PLUS 5d6 bonus assassination damage dice). The attack has slightly more than a 50% chance of killing the equal level monster outright, and it deals 12 HP more on average than a failed assassination attempt under the old rules. Against equal level opponents, the houseruled variant is actually more powerful.
As you run the numbers against foes that are higher leveled, the percentage chance of killing the target outright actually drops much faster than it does as written because monster HD are d8 and the bonus damage dice are d6. Lets look at a 10 HD creature, outclassing Vivuli by 5 levels. This would be a 25% chance of a kill as written. A 10 HD creature has 45 HP on average. Vivuli's maximum damage is 39. He could kill a 10HD creature outright only if the creature had a poor HP roll. But again, on a failure he still does 23 HP damage on average, which is half the creature's HP.
Now let's look at the way the d30 houserule interacts with both systems. Used as an attack roll, it obviously gives a much better chance of hitting on the initial backstab attack. The d30 is useless on the actual assassination attempt roll as written, because it is a d100 mechanic and the d30 only goes to 30. However, the d30 roll used in conjunction with an assassination attack under the houserule opens up the possibility of a critical hit on a 20 or higher on the d30, which does double damage. This means his average damage would be 46, roughly a 50% chance of killing a 10HD monster. It is also 24 HP more than the result of a critical hit on an assassination attempt that failed to outright kill the target under the original rules (average damage 22 HP on a failure, base backstab x2).
I don't think the houseruled variant is any more or less "powerful." It is just flat out different, so you can't really just compare the probabilities of a kill between the two. It is more reliable, as its damage output is the same even if it fails to kill the target, and it interacts nicely with the d30 to give Danny a once a session 33% chance to do an average of 46 damage (and obviously if he gets a good 6d6+1d3 roll he could do considerably more). It does not give a chance of killing outright foes that are much higher level unless they have unusually low HP.
But that is where this gets REALLY tricky. This whole analysis has assumed that Vivuli is targeting a monster. He can also target fighters, magic users, etc. Against a wizard, the new variant has a decided bonus as the wizard has much fewer HP proportional to level than other classes. Against other assassins, rogues and clerics, the variant has a slight edge in percentage chance to kill across levels as both extra damage dice and HD are d6, but the initial attack also adds 1d6+1d3 to it.
Anyway, enough of that bullshit. The long and the short of it was that the entire point of the houserule was to do away with the ability to kill outright much higher level creatures because Carter found it so disruptive of his immersion in the game that it stopped play several times when it occurred. The houserule is a compromise that allows Vivuli an assassination ability that is useful in play without the chance of autokilling much higher level opponents.
That being said, I know Danny much prefers the original way, and I think I do myself. But I'm not DMing this game...
Thanks Carl! I imagine some further discussion will follow, but right now I am feeling like I have inexplicably made peace with the assassin as writ in the AEC. bdfiscus' comment on the original assassination houserule post supports this position, suggesting that I "leave the class as written and either allow or disallow the entire class." He says:
If you mislike the assassin's chance to insta-kill the "boss monster" with a surprise attack then give the demigod, devil, or boss a special caveats such as "immune to surprise" or "hit only by +"X" weapons"; or the creature in the encounter is already alert and ready for combat and there are penalties to the surprise roll; etc.
I personally - yes, even as a GM- actually LIKE the "all or nothing" ability of the assassin.... and that ability cuts both ways (pun intended) since NPC assassins have the same chance to "one-shot" the PC's as well!
Agreed; despite my earlier complaints and decree, I feel like am now willing to just go with the flow and allow assassins to kill targets instantly. Perhaps it is a matter of expectations: back when Vivuli insta-killed one of my saurian balrogs in Session 50, I wasn't expecting it; whereas last session with the magma creature, I was, and I actually missed the "all or nothing" insta-kill when it didn't happen.
Sorry folks, clearly I was the asshole on this one.