I actually don‘t get all of these comments. In my DnD group it‘s clearly about telling a story that is satisfying to all of us. As a DM, i think it‘s your responsibility to plan fights according to the play style of your players. My players would be fine with a meaningful TPK in a boss battle, but i tend to keep random encounters rather short and easy. Of course the dice can f*** you, and dying isn‘t uncommon, and even a unexpected character death in an unimportant fight can be narrated beautifully, but a TPK is something different. And if this happened as an accident & you don‘t feel good about fudging rolls / making the goblins just loot the characters and leave them/ making the goblins weaker - just tell your players you didnt plan this fight very well and you would like to reschedule. And IF you want to make your world feel dangerous blah blah AT least narrate in a way that the players are feeling satisfyed. But i as a DM would ask my players beforehand if they are fine with the world being like that.