Look, I get it.
It sucks to have to kill your PCs.
(Well, it usually sucks.)
Any DM worth their salt knows how much time, effort, and emotional investment players put into their D&D characters, especially after months, or even years, of playing them in an ongoing campaign.
But you cannot be afraid to kill them.
Doing so robs the entire party of one of the greatest things about Dungeons and Dragons… the fear of death, the knowledge that their actions have consequences, and the very real threat of someone they’ve come to know and love being gone for good.
- See also:
Too many DMs will alter the wills of fate (AKA the dice) in order to keep characters from suffering an untimely death. The moment this happens, players around the table will begin to realize that destiny is on their side…. which could remove any tension from otherwise deadly encounters.
Why bother stealthing through the enemy camp if the DM will bend the story over backwards to avoid a Total Party Kill? Why worry about healing your downed ally if you know the DM will allow advantage on death saving throws if they ever come close to three fails?
The worst thing that a DM can do is allow the party to believe that they are invincible. That’s how murderhobos happen, and worse, that’s when boredom sets in.
When the only thing that the players have to worry about is how they’re going to dispatch the next threat, or how to steamroll their way to victory, players can lose interest in what’s even going on.
So introduce that Level 20 retired adventurer NPC with a glib tongue. Bring it to blows if/when your party mouths them off.
Make high CR areas and monsters accessible to the party at lower levels, but make it clear that they will die if they try to enter those areas at the power level they are now.
Make a show of force in the early levels to show that death is a very real possibility in your world.
And watch as your PCs factor that truth into every encounter for the rest of the campaign.
Because the only thing worse than killing a character is letting them live when they very well should have died, and the Pandora’s Box that doing so opens up for the entire party.
But that’s just my two coppers.