Durin, the great abyssal dragon who's corpse litters Dragonspine (hence the namesake). Who's heart still beats centuries after his defeat. A creation of Rhinedottir that "attacked" Mondstadt during the events of the cataclysm. Who's fight with Dvalin alongside Barbatos (Venti, the anemo archon) left Dvalin poisoned for 500 years leaving him vulnerable to influence from the abyss order. The threat that is Durin continues to loom over Dragonspine and Mondstadt as even though he lays dismembered strewn across the mountain blood flows from his still beating heart. One day he may restore himself and "attack" once again. The presence of his corpse makes Dragonspine a harsher place as his blood alone has altered the life on the mountain and attracts unwanted attention. He's was an abyssal monster who's appearence worsened an on going crisis.
Similarly over in Fontaine there was the abyssal dragon Elyans. Another creation by Rhinedottir that also layed waste to Fontaine during the cataclysm. What's interesting about Elyans comapred to his brother Durin is that we get to learn more about and communicate with him. From both this and his offsprings (the Melusines) we learn that those of the abyss like Durin, Elyans, and even the Melusines see the world differently. Differently enough that Melusines have historically had a hard time integrating with Fontainan scoiety. Creatures like rifthounds (another creation by Rhinedottir (on gawd why she make those, they annoying, got the whole party on their knees even if they haven't seen the field)) appear to them like dogs and act accordingly (This is in a Fontaine world quest). So for Elyans all those years ago when he finally got to travel Teyvat and meet the life that littered the world, he'd eventually come to realise that his traveling brought suffering. His existense alone harmed others where he desired companionship resulting in him seeking death so he couldn't hurt anyone.
Durin was similar to his brother Elyans, Like Elyans he perceived the world differently, but unlike him never came to the realization that what he perceived differed to reality. At the same time when he appeared in Mondstadt all those years ago he apparenty was poisoned, ill from consuming subject two (A prior failed itteration of Albedo, another creation of Rhinedottir). In what little we do get from him he speaks of his "attacking" of Mondstadt and following death. He describes the events as being drawn in by the soulful songs (probably from Venti, or not potentially due to the perception difference. Maybe the songs were the screams of the people during the cataclysm. I mean Venti only awoke because of Durin's attack, not the cataclysm (this is a lie, he was summoned for Khaenri'ah)), singing along the people, and finally dancing in the sky with "a dragon as beauteous as a jewel". All this as "quoted" from the Dragonspine Spear. His view of events clearly don't match reality but they still allow you to undertand him. There's a desire for companionship thats present with him, something he doesn't live long enought to obtain. The skyward blade, pride, and spine all give a more accurate telling of events; his appearence in Mondstadt, his fight with Dvalin, and the tearing of his throat by Dvalin. (this was helpful a hoyolab post by Mlem000)