This is based on opinion. Please don't list it on a work's trope example list.
Tear Jerker / Legacy of the Dragokin
Daniar and Kalak's marriage falling apart is awful because they were last scene in traditional Happily Ever After at the end of Trapped On Draconica. This sense is componded because Ritchie's death between books set it off.
Learning that Daniar's firstchild died in her womb because her opponent stabbed her there.
Lydia's teddy bear from childhood is charred from a fire. This is because her home caught fire years ago and her mother died retrieving the toy.
As evil as they are one must feel sympathy for the Kthonian Knights:
Their backstories involve kidnapping, rape, dismemberment, and deliberate humiliation For the Evulz.
Mordak killing Furie is heart wrenching for two reasons: 1.) he's eviler than the knights and 2.) his victim was desparately trying to prove herself to her family and her 'brother' really is her brother and went ape shit when she died.
Discovering Kthonia (her mother and idol) is not the man-eater she remembered Jihadain morale is crushed. It had the effect of turning her world upside down and proving everything she believed in was a lie. Dying by Kalak's hands makes her happy because her mother reverts back to her old ways and thereby vaildates her life's work.