1st level Barbarian goes down. A wily Paladin lays hands on him, brings him back to 1 hit point out of 13 thanks to his CON +1.

A Hobgoblin doesn't like that he was revived, so he slashes with a longsword for 6 damage. The Barbarian is down _again_ (he didn't have a chance to rage because he was just revived... turn order and stuff)

But then - here's the kicker... Martial Advantage deals another 8.

From the PH:

Instant Death

Massive damage can kill you instantly. When damage reduces you to 0 hit points and there is damage remaining, you die if the remaining damage equals or exceeds your hit point maximum.

So how do you rule?
A) He had 1 HP, he took 14, his max is 13, he'd be at -13 ... Instant Death
B) He had 1 HP, he took 6 taking him down. Then, while incapacitated he took 8 more, that's going down 2 death saves
C) You have some other clever alternative that I didn't think of

(At the table I ruled B)... and with some fast negotiation with the pirate captain, the Barbarian was stabilized to fight another day)

Did I make the right choice? Miss on the ruling? There was plenty of fun and laughter and shouting, so at the table much fun was had - so I call it a win in any case. But I am curious what the "right" answer should've been. Thanks for taking the time to read and throw in your 2 cp.

The extra damage is part of the weapon attack, so it's 14 damage, instant kill.

It's a bit unclear where an attack has two somewhat separable attacks (e.g. an imp reduces someone to 0 hp with a sting, and then applies additional poison damage), but martial advantage is like sneak attack -- it adds to the damage of the attack that triggers it, rather than being separate damage.

As stated by the others, the rules say that the barbarian is dead dead.

But, you asked if you made the right call, and following the rules isn’t always the right call. I think it’s unreasonably unfun to die at 1st level (unless it’s at a dramatic moment at the end of a one-shot; it’s happened to me). I don’t think there’s anything wrong with creatively misinterpreting the rules even on purpose. It sounds like you sincerely made a call you felt was reasonable at the time (which is almost always preferable to slowing down the game to look something up). Everyone seems to have been happy with the outcome, so I’d say you made the right call.

In the last while, I’ve been treating my goal as a DM is to get my PCs to level 2 in any way possible.  That’s it.  I don’t worry about finding a tough enough challenge, or enforce strict rules about where they can rest, or make the dungeon harder if they leave and come back… my only goal is fun and level 2.

Then I crush their hopes and dreams at level 2 onwards. 

Quote from Brewksy >>

In the last while, I’ve been treating my goal as a DM is to get my PCs to level 2 in any way possible.  That’s it.  I don’t worry about finding a tough enough challenge, or enforce strict rules about where they can rest, or make the dungeon harder if they leave and come back… my only goal is fun and level 2.

Then I crush their hopes and dreams at level 2 onwards. 

Yeah I treat first level more as "tutorial mode."  Even if it's an experience group, I use the level as a space for the characters to figure out how they gel.  Challenges/encounters will not usually not be lethal, but their performance will have consequences to reputation/standing, things lime that.

Chop of his arm instead. Permanent reminder of that nearly fatal lvl 1 encounter.

The "right" answer was A, but you made the right choice as a DM.

RAW it would result in an Instant Death. Multiple Hobgoblins ganging up on a proned 1st-level character with 1 HP  can be deadly due to Martial Advantage. I think you did the right choice. Another good choice would have been to distance Hobgoblins by having only 1 within 5 feet of the Barbarian so it doesn't benefit from Martial Advantage against him.

Thanks for the responses!

I thought about it over night and actually, I messed up twice! The hobgoblin advanced on the Barbarian because he was threatening the boss. So the Hobgoblin was flanking the Barbarian with his boss. The were NOT within 5ft of each other. So Martial Advantage didn't even apply. But he would've had advantage since we are playing with the optional flanking rules.

It was crazy chaotic. The boss wizard lost a few fingers... he was not pleased.

Moral of the story, the party's 2nd level, but they were forced to submit and are at the mercy of the angry 8-fingered boss wizard, and his hobgoblin lieutenant in the caves under the haunted mansion in Saltmarsh - oh, the possibilities.

Quote from MyklK >>

Thanks for the responses!

I thought about it over night and actually, I messed up twice! The hobgoblin advanced on the Barbarian because he was threatening the boss. So the Hobgoblin was flanking the Barbarian with his boss. The were NOT within 5ft of each other. So Martial Advantage didn't even apply. But he would've had advantage since we are playing with the optional flanking rules.

It was crazy chaotic. The boss wizard lost a few fingers... he was not pleased.

Moral of the story, the party's 2nd level, but they were forced to submit and are at the mercy of the angry 8-fingered boss wizard, and his hobgoblin lieutenant in the caves under the haunted mansion in Saltmarsh - oh, the possibilities.

No your ruling was correct. The  target must be within 5 feet of the Hobgiblins and it's ally, not that the Hobgoblin and it's ally be within 5 feet of each other.

Martial Advantage: Once per turn, the hobgoblin can deal an extra 7 (2d6) damage to a creature it hits with a weapon attack if that creature is within 5 feet of an ally of the hobgoblin that isn't incapacitated.

TLDR I’m pretty the rule means twice your hit point maximum.

Quote from monkmulticlasser >>

TLDR I’m pretty the rule means twice your hit point maximum.

 The rule is twice your hitpoint maximum *total*, if you were assumed to be at full hp at the start. It's not twice your hitpoint maximum after you drop to zero. 

One more suggestion ... I'd drop the use of the flanking rule. Giving advantage is a huge benefit that enables a number of other abilities and tends to just make things more deadly. If you really want to give something for positioning, I would suggest a +1 if you are in a flanking position - not advantage.

P.S. As noted above - Martial Advantage only requires an ally within 5' of your target - not within 5' of you. As you described it, the hobgoblin and the BBEG were both within 5' of the barbarian and as a result the Martial Advantage would apply.

P.P.S. Just agreeing with the rest - RAW they would be dead dead but it was fine the way you ran it since it is never fun dying at level 1. Also, I'm not sure how you assigned stats but barbarians might typically try for a con higher than 12.

Correct... if you reach the negative value of your max HP in a single blow, you die. If your max HP is 40, then any single blow that would mathematically put you at -40 kills you. At 40 hp, it would take 80 damage to kill you in one shot. At 20 hp, it would take 60 damage (20 - 60 = -40). And at 1 hp, it would take 41 damage (1 - 41 = -40).

We actually rolled our characters using the Discord Avrae bot. So we have a Barbarian with a 12 CON, but an 18 STR! (We also have a Paladin with a FIVE Strength, he's struggling a bit)...

It's actually kind of fun to play against your weaknesses. Unarmed strike with a -2 to damage. (I know, it's actually 1, but still funny)

Quote from David42>>

One more suggestion ... I'd drop the use of the flanking rule. Giving advantage is a huge benefit that enables a number of other abilities and tends to just make things more deadly. If you really want to give something for positioning, I would suggest a +1 if you are in a flanking position - not advantage.

Seconding this. It also devalues the typical means of getting advantage, especially limited-use abilities like spells and class features. In my opinion the advantage/disadvantage system falls apart if you add a constant source of one or the other.

Quote from MyklK >>

It's actually kind of fun to play against your weaknesses. Unarmed strike with a -2 to damage. (I know, it's actually 1, but still funny)

It's actually 0. 1 + STR mod, and while there's no such thing as negative damage there's also no minimum of 1 damage in the rules.

Quote from VanZoeren >>

Quote from monkmulticlasser >>

TLDR I’m pretty the rule means twice your hit point maximum.

 The rule is twice your hitpoint maximum *total*, if you were assumed to be at full hp at the start. It's not twice your hitpoint maximum after you drop to zero. 

 Thanks for the clarification :)

It's your table you can do whatever you want and you will be right no matter what (as long as you don't do anything super outside the rules)

It's tricky to figure it all out.  You have a 1st level Barbarian with 13 hp.  I'm going to call this character "Bob".  Hi Bob!

• At first level, it takes  26 hit points done in one shot to flat out kill poor Bob.
• If Bob only has one hit point, and takes 14, it's time to take that first Death Check
• If Bob only has one hit point, and takes 5, he would need to take that first Death Check, but wait!  There is more!
• Bob got hit for 8 more.  The rules say that no matter how many times they take damage, their hit points don't go below zero.  Time for a Death Check.
• Good thing that Hobgoblin didn't get a crit! 2d8+4d6 averages 23.  They only need to do 3 more, and with a minimum of 6 and a max of 40, Bob is probably Dead, Dead, Dead.

I'll send flowers for Bob, or whatever name they really had.