Down but not out rule:

When a character goes down to zero HP the following comes into play:

1st time down

DC5+con exhaustion roll

Failure grants 1 point of exhaustion - continue death saving throws as normal

Success no exhaustion gained - continue death saving throws as normal

Crit Failure - 1 point of exhaustion - roll death 1st death save with disadvantage

Crit Success - no exhaustion - roll death saves with advantage

DC increases by 5 for each time reduced to 0HP before taking a long rest.