Reiner did die in the manga. His death was a result of a series of events and conflicts that had been building up throughout the storyline. It had a major impact on the plot and the other characters.
If Lord Reiner is from a particular book or game, more context is needed. For example, if it's a character from a fantasy novel, we would need to know the plot of the novel, the world it is set in, and his relationships with other characters to tell his full story.
There could be stories about how he dealt with new musicians or young talent. His high standards might have been overwhelming for those just starting out. He expected them to quickly adapt to his way of working, and if they didn't, it could lead to some rather unpleasant experiences for the budding musicians.
Love and sacrifice are common themes. Often, Reiner has to sacrifice something for the sake of love in these fanfictions. It could be his position in the Legends world or his own safety.
I'm not sure specifically which 'Fritz Reiner horror stories' you mean. It could be about his strict conducting style in the orchestra. Some musicians might have found his high standards and no - nonsense approach intimidating, like he would demand absolute precision from every player.
Some fanfics also focus on the psychological aspect of their relationship. Reiner and Bertholdt both have complex personalities and backstories. Fanfics might explore how they understand and support each other on a psychological level, perhaps dealing with their inner demons together.