How much damage does the NPC assassin do in the first two rounds to a sleeping creature?

Question

A related concern in calculating the damage potential is does the poison stay on the short swords after the first round? I am trying to get a grip on how much damage does the NPC assassin does in the first two rounds to a creature who begins the encounter asleep. There are a couple of assassination attempts looming in our campaign that I need to get a better feel for. (This related question got me thinking).

Given

1. NPC Assassin attacking first in a round (two shortswords, Multiattack)

2. Target is sleeping (unaware of surroundings)

3. A successful Dexterity (Stealth) check allows the Assassin to have surprise (but target wakes up if still alive once the attack happens).

4. Assassin can expect additional opposition in Round 2, and needs to finish off the target to get out for a high percentage chance of mission success.

5. Use average dice scores for ease of computation, since so many dice are being rolled

Find

a. The nova damage for the NPC Assassin in Round 1.
b. The damage in round 2

Relationships

Assassinate. During its first turn, the assassin has advantage on attack rolls against any creature that hasn’t taken a turn. Any hit the assassin scores against a surprised creature is a critical hit.

1 Sneak Attack (4d6), 2 hits with short sword, two saves versus poson.

Shortsword. Melee Weapon Attack: +6 to hit, reach 5 ft., one target. Hit: 6 (1d6 + 3) piercing damage, and the target must make a DC 15 Constitution saving throw, taking 24 (7d6) poison damage on a failed save, or half as much damage on a successful one.

This is for me the tricky part. I don't think the poison gets doubled due to having a saving throw, similar to a giant scorpion's stinger. (Poison's a rider). It isn't clear to me that the poison stays on the shortswords once the first attacks/hits with the short swords are completed.

Key issues to resolve:

Is the poison still on the blades in round 2?
Is the target prone (attacked with advantage) on round 2? (For reasons to answer "no" to this, please explain)

What I arrived at:
Round 1 2x((2d6) + 3) + 2x(4d6) + 14d6 (Assumes neither DC 15 con save is made)
Round 2 (no crit assumed): 2x((1d6)+3) + 4d6 + 14d6. (Assumes neither DC 15 Con save is made)
That gets me to (26d6 + 6) + (20d6 + 6) = 46d6 + 12 = 173. (Each Con save versus poison reduces this by 12.25). Four saves drops it to 32d6 + 12 = 124.
The proposed target has 112 HP, which means the result is 'insta-kill' unless the die rolls are horrible on four attacks with advantage.

What I need from an answer

1. Have I correctly calculated the damage done in two rounds if both attacks hit in each round?

2. Is the target remaining prone for round two a poor, or a valid, assumption?

3. Does the poison stay on the blades for the second round's attacks?
   If it does not, the numbers reduce significantly.

Answer 1

1. Damage depends on initiative.

Its important to remember that ALL the creatures roll initiative, even if they aren't aware of the dangerous situation, this sets them on when they can act once they are aware.

From PHB under Combat:

When combat starts, every participant makes a Dexterity check to determine their place in the initiative order.

Being a target makes you a participant, whether you want it or not. Furthermore, under conditions under unconscious:

• An unconscious creature is incapacitated, can't move or speak, and is unaware of its surroundings.
• The creature drops whatever it's holding and falls prone.
• The creature automatically fails Strength and Dexterity saving throws.
• Attack rolls against the creature have advantage.
• Any attack that hits the creature is a critical hit if the attacker is within 5 feet of the creature.

As bad as being unconscious is, it does not remove you from initiative (or even give disadvantage)

Also important: From Sage Advice Compendium about being Surprised:

For triggering the rogue’s Assassinate ability, when does a creature stop being surprised?

After their turn in the round, or at the end of the round? A surprised creature stops being surprised at the end of its first turn in combat.

For further reading, there are other important questions regarding initiative and surprise:

surprise can act oddly if the target rolls high initiative.

See a related question about surprise here.

2. Depends on how initiative was rolled by both creatures.

If a creature is surprised, it is prevented from acting until the end of its first turn. When combat began, both characters rolled initiative. The sleeping character may have rolled higher than the assassin. However he didn't act as he was surprised (and asleep). However at the end of his turn, he can make reactions and he can take actions (on his next turn). Now the assassin still gets to attack, but the target isn't surprised. So when they take damage, they could technically even react to it, as the damage woke them up.

Second round, the target gets to go first, so he can stand up as normal.

3. Poison Stays.

For point 3. NPC abilities (while often named similarly to items and equipment) do whatever they say they do. Nothing more or less generally.

The assassin attack says:

Shortsword. Melee Weapon Attack: +6 to hit, reach 5 ft., one target. Hit: 6 (1d6 + 3) piercing damage, and the target must make a DC 15 Constitution saving throw, taking 24 (7d6) poison damage on a failed save, or half as much damage on a successful one.

Compare this to say a giant poisonous snake:

Bite. Melee Weapon Attack: +6 to hit, reach 10 ft., one target. Hit: 6 (1d4 + 4) piercing damage, and the target must make a DC 11 Constitution saving throw, taking 10 (3d6) poison damage on a failed save, or half as much damage on a successful one.

What about the first one implies it would stop after an attack?

Answer 2

Your numbers are correct if all attacks hit

Assuming that your given #1

NPC Assassin attacking first in a round (two shortswords, Multiattack)

means the assassin is first in initiative order, he will get the full assassinate damage in round 1, and will still act first in round 2.

From your given #2, the target is unconscious at the beginning of combat, and thus prone. From given #3, the target is surprised and as such does not move or act on the first round of combat, even if awoken by the assassin:

If you're surprised, you can't move or take an action on your first turn of the combat, and you can't take a reaction until that turn ends. A member of a group can be surprised even if the other members aren't.

Standing up from prone requires movement:

Combatants often find themselves lying on the ground, either because they are knocked down or because they throw themselves down. In the game, they are prone . You can drop prone without using any of your speed. Standing up takes more effort; doing so costs an amount of movement equal to half your speed. For example, if your speed is 30 feet, you must spend 15 feet of movement to stand up. You can't stand up if you don't have enough movement left or if your speed is 0.

Because they are surprised, the target is unable to use movement to stand during round 1, and remains prone at the beginning of round 2. As you act first in initiave order and the target has not yet had the opportunity to stand, you still attack the prone target with advantage and can sneak attack, though you do not get the automatic critical hit.

As for the poison, if an effect is listed as part of an attack in a Monster Manual stat block, it does not go away unless explicitly stated so. Therefore in this case, the poison effect would trigger on every hit of the shortsword.

In conclusion, your numbers of

Round 1 2x((2d6) + 3) + 2x(4d6) + 14d6 (Assumes neither DC 15 con save is made)
Round 2 (no crit assumed): 2x((1d6)+3) + 4d6 + 14d6. (Assumes neither DC 15 Con save is made)

are correct. Two critical shortsword hits and one critical sneak attack, with two doses of poison in round 1; two shortsword hits and one sneak attack, with two doses of poison in round 2. This would come out to (26d6 + 6) + (20d6 + 6) ≅ 173 damage on average.

Using the Dungeon Master's Guide poison rules

The DM may disagree that the poison persists and should instead follow the Dungeon Master's Guide rules on injury poisons (DMG page 257, emphasis mine):

Injury. Injury poison can be applied to weapons, ammunition, trap components, and other objects that deal piercing or slashing damage and remains potent until delivered through a wound or washed off. A creature that takes piercing or slashing damage from an object coated with the poison is exposed to its effects.

In this case, the poison would only take effect in round 1 of the combat for damage values of:
(26d6 + 6) + (6d6 + 6) ≅ 124 damage on average.

This is possibly enough to still kill your target if they fail both CON saves and you roll average or above on the dice, though it is less of a sure thing.

---

More accurate numbers

The above calculations assume that every attack hits, below I will go into more detail using the odds of an attack hitting. I believe you mentioned in a comment or in chat that you are planning to attack an NPC Gladiator, so I will be using that as my example.

For the sake of these calculations I will assume the gladiator is sleeping without its studded leather armor or shield for an AC of 12, and no weapon in hand (so it can't use the parry reaction in round 2).

The gladiator has a +6 CON save, which gives a 40% chance to fail the poison's DC 15 saving throw for 24.5 damage (7d6), or 60% for half damage of 12.25, giving us an expected output of (0.4 * 24.5) + (0.6 * 12.25) = 17.15 poison damage per attack that hits.

The assassin has a +6 to hit with the shortsword, with advantage this translates to a 93.75% chance to hit AC 12 (84% chance of rolling 6-19, 9.75% of natural 20). The average damage of a critical hit is 27.15 (2d6 + 3 + poison), the average damage of a non-critical hit is 23.65 (1d6 + 3 + poison). There is a 99.6% chance of hitting at least one attack in a round, giving us sneak attack for 14 more damage. These numbers give us the following damage output:

Round 1: 2 * ((0.9375 * 27.15) + (0.996 * 14)) ≅ 78.8
Round 2 (with poison): 2 * ((0.84 * 23.65) + (0.0975 * 27.15)) + (0.996 * 14) ≅ 58.9
Round 2 (without poison): 2 * ((0.84 * 6.5) + (0.0975 * 10)) + (0.996 * 14) ≅ 26.8

This gives us a total estimated damage of about 137.7 if the poison persists in round 2. Assuming dice rolls are around average, this should result in an instant kill most of the time against a gladiator's 112 hit points.

If the poison is one-time-use, the estimated damage lowers to around 105.6, and would not be quite as likely to instant kill the gladiator.