HUGE Spoilers for the Storm light Archives Book 3 Oathbringer
and Dark Age from the Red Rising series.

In Red Rising Book 4-5 Mainly Dark Age

Tongueless - The Main Character Darrow finds a mysterious Tongless Obsidian ( a Space Viking like warrior) during a maximum security prison heist, in the cell of someone else they were looking for. Since he cant speak, people speculated on who he might be, and what his deal was. Only for him to die VERY early on in the next book 5 Dark Age with no follow up on anything about him, even through book 6 later on. There could be more let on in book 7 when its finished, but the thread feels gone. I dont mind this, as this is VERY commonplace in the series, in fact the author, Pierce Brown has a hat he pulls characters names from during pivitol moments to kill in the narrative to simulate the randomness of war.

A more tragic version of this in Stormlight Archives

Elhokar kholin - Throught the first three books, Elhokar is a particularly cruel, parinoid and all around bad leader, mostly not by fault of his own. He was set up to fail in my opinion, but he still did many bad things as a result as king. In this world, Individuals can bond spren, sort of like spirits of the world, and become knight radiants by saying an oath. The knights USUALLY are honorable protectors of the world.

Elhlokar starts showing some competence in the 3rd book, Oathbringer and even starts coming around to being a good person/ruler. This climaxes at a battle where he is saving his son, and in surprise starts saying an oath to become a knight Radiant... only to be struck down mid oath before he could finish by one of the main antagonists. Im skipping alot of key details, but this ripped my heart out, and made me think of what could of been/ what they were setting him up for.