This is based on opinion. Please don't list it on a work's trope example list.
Tear Jerker / Sands of Destruction
Kyrie discovering that he is the Destruct, the key to ending the world. And this comes right after it's made clear that he has feelings for Morte, and they had almost reached an agreement at the peace talks with the human rebels and the Beastlords which would allow Kyrie to spend more time with Morte. Ouch.
When Kyrie asks Naja to kill him, just to prevent the end of the world. Kyrie then expresses his thanks when the Beastman complies, to Naja's astonishment and later regret.
When Kyrie tells Morte he loves her just before he dies, leading Morte to suffer a Heroic B.S.O.D. for several days before she realizes there is a way to bring him back.
The magnitude of said Heroic BSOD is further pushed if you look up Morte's bios after that: