anonymous asks: In The Sunken Tomb (44) Vex dies instantly from being hit with enough Necrotic damage to take her down to 0. But, in Whispers (29) Percy also goes down to 0 from the Banshee's slash that deals Necrotic damage, yet doesn't die. Is it a rule in 5e that Necrotic damage that takes you down to 0 is an auto-kill? Because I seem to recall it was the same way during the K'varn fight with Grog, and that was also the fear at the end of the Ziggurat fight? Or maybe Matt just forgot about it?
Necrotic damage, by itself, is simply a type of damage. As anti-life energy, it is harmful to living creatures, but may have little or no effect on undead or constructs. Many attacks that use necrotic damage also have an instant death effect tied to them, but dealing necrotic damage to a player character with low HP isn’t necessarily the kiss of death for them.
From the non-instant-kill side, you bring up the Banshee’s Corrupting Touch (average of 12 necrotic damage). A PC who fell victim to Keyleth’s Blight would not die automatically, either (unless they were somehow a nonmagical plant, as explained in the spell description). Beholders also have a non-instant-kill necrotic eye ray called the Enervation Ray (36 on a failed, half on save).
However, the attacks you mentioned above all have some sort of death effect tied to them. There’s the Death Ray that Beholders have (55 on a fail, half on success). The spell description for Finger of Death explains that victims who drop below 0 HP instantly die and become a zombie under the control of the caster on their next turn. The trap surrounding the champion of the Raven Queen (the goddess of death) also killed those who fell below 0 HP.
So, necrotic is bad news, but really, whether it’s instant or not depends on the spell description.