Yep, just know. For the first game, complete I believe 20 quests before Chapter 9 I think it was. If you don’t do this, you lock yourself out of the True End.
Quests are very simple, and this is insanely unlikely to miss.
As for the general premise. Death End ReQuest. It follows how the characters continuously die until they break out of the cycle.
Hence the Death Ends. It is hard to describe without spoiling it. Think of it more as a failed world that is barely surviving due to its resets, and is only narrowly escaping erasure from the skin of their teeth.
A world plagued by bugs, and another lifeform that watches over countless worlds, acting more like dictators. Erasing worlds that are failures, this being towards their utopia.
Have you played Anonymous;Code? The premise sounds similar to how you described Death Ends. The beginning has a tasteless ’joke’ otherwise it is a fairly good game if you are up for trial and error.
Without a guide there are moments that are difficult to tell what to do to avoid a bad end. Usually it hits you over the head for when you need to make your choice, but sometimes it is so subtle that it is quite easy to miss.
When the game starts off as well. You are shown a Death End. This is a clue for when you reach the part of the game. In short, it is telling you to prepare far ahead of time.
In Anonymous;Code the game starts with a temporary good end then goes into a flashback on how to turn a bad end into a good one. Hard to describe without spoilers.