5e you can only drop to zero, at which point you start making death saving throws. Three successes, you’re “stabilized” still at zero hp and unconscious, but alive. 3 failures, you die. If you get hit while at 0 hp, it counts as an automatic failed death saving throw. If you get a natural 20 on a death save, you regain consciousness and are back at 1 hp. Give me a sec, I’ll get you a link for it.

(By 5e, you drop to zero unless a single hit equals or exceeds your max HP. So no negatives. So the critical save restores him to 1hp)

<<<

Dropping to 0 HP: If you take enough damage to drop to 0 HP, and the remaining damage equals or exceeds your HP maximum, you instantly die. If the remaining damage is less than your HP maximum, you fall unconscious until you regain HP.

Each time you begin your turn with 0 HP, you need to take a death saving throw (d20). A ten or higher is a success; below a 10 is a failure (Criticals count as 2 throws). After three successes, you become stable (see below), but on your third failure, you die.

If you take damage after initially dropping to 0 HPs, it counts are a throw failure. Critical damage counts are two failures. If the damage equals or exceeds your HP maximum, you die instantly.

In order to wake up, you need to regain HP, either by being healed or gaining a HP for each 1d4 hour you spend stable.

Becoming Stable: A creature that is stable has 0 HPs but does not need to make death saving throws unless it takes damage again. Three successful death saving throws or a successful (DC 10) medicine check by another creature renders the creature stable.

*Note: Upon waking up from 0 HP, a creature is prone (lying on the ground). The creature must use half its movement to stand up (stop being prone), otherwise it has to crawl. While prone, a creature has disadvantage on attack rolls. Attackers have advantage on attack rolls if they are within 5ft of the prone creature, but disadvantage if they are farther away.>>>

(Jinx)

OK... So, you are at 1 HP.  And Help is on the way....

"We need of of the holy types, I'll go get Cellica and Darwin."

Running downstairs, Wren looks around, surprised not to find the rest of the group. "Darwin? ... Cell? ... Terrance?" He goes into each of the downstairs side rooms, scratching his head. "Bloody idiots must have gone into the fog..." he heads too fhe front door preparing to shout, opens it and... Sees Darwin?

Horus coughs and begins to shiver. “I think this isn’t going well. One more like that and I’m done for. But we have to continue, who knows what’s happening to the others the longer this thing persists.”

Quote from Renegado_ >>

"We need of of the holy types, I'll go get Cellica and Darwin."

Running downstairs, Wren looks around, surprised not to find the rest of the group. "Darwin? ... Cell? ... Terrance?" He goes into each of the downstairs side rooms, scratching his head. "Bloody idiots must have gone into the fog..." he heads too fhe front door preparing to shout, opens it and... Sees Darwin?

The door was left open, so Darwin would be just stepping out of the fog and stopping abruptly.  

((My bad, I misread the Darwin gets back entry))

"Darwin! We need you upstairs, Horus is hurt!"

Wren starts pulling his arm, urging him towards the stairs.

Horus

 <<  Let's assume Darwin heads upstairs.. not sure if he is paying attention >>

Horus moves to the top of the steps to have a look at what’s going on downstairs.

”Darwin?? Thank the... did you find the others?”

Darwin is at a loss of words after what had just transpired, of all things the Wildmother could send to aid him It was a rat. Stepping into the Tavern just in time to see Wren. "Wr-" He is caught of guard when Wren attempts to pull him forward (Not happening, Darwin is like 300 Pounds of muscle) 

"Horus is hurt? What happened?" Darwin rushes up the stairs to see Horus. "Cellica and I went in search of Terrance, Unfortunately we got separated in the fog." Darwin can obviously see Horus is in bad shape.

Darwin will use Lay on Hands and give Horus 5 Hit points. (I cant heal yet, Level 2 before I get my spells.) 

Horus clings to Darwin for a moment, it’s almost embarrassing. “Thank you, Darwin. Thank you.” He then straightens up. “Ahem. Let’s finish this. Andolin? Lead the way?”

Andolin

“Let’s go. The dragonborn and I will take point, if he’s willing. This may be the only way to rescue Terrence and Cellica, short of divine intervention.”

Andolin leads the others to the door to room 6, and will open it when all are ready.

Cell just continues to wander aimlessly around in the fog looking for both Terrance and Darwin when she hears a voice. She turns around but can't pinpoint the location of the voice exactly "Who's there?" she asks and holds her staff at the ready again.

<<@Andolin, or when the DM wakes up>>

Once again, this door is locked.

Cellica

Hey guys, imma bow out of this campaign. I've been stuck in the fog with nothing to do for 8 days I r l with no checks or anything to do. It's not very fun.  

The nail in the coffin is that the only way to get our is if the dm just randomly decides.  So yeah peace out. 

Wren will attempt to unlock the door.

Thieves tools 14

((Sorry too see you go Hazard))

Wren unlocks the door to reveal, a 20’ by 20’ room features two single beds with a nightstand between them, one table and two chairs.  This is the room of the two merchant guards. They are both in bed with the covers up, their faces unnaturally pale in the light.  Their swords rest on the floor beside them.

Cellica runs back inside the tavern and quickly looks around if she can see the others, then heads towards them as they open the door to another room. "Hi there, so the weirdest thing happened, A raven lured me back to the tavern and then flew away screeching something that sounded an awful lot like "Flee" .. I think we should find a way to get out of here and we should find it fast." she said still catching her breath from running up the stairs towards the group.