Lore similarities between DD and Dark Souls - A Great Cycle to sustain the world
The main theme that connects these two worlds is that of a great "Cycle".
In DD, it is the cycle linking the Dragon, the Arisen, and the Seneschal. More so, some dialogue from DD implies that as time passes, the Seneschal's will is what sustains life in the world and it becomes depleted, negatively affecting the nature of living beings. To "renew" this, another Arisen must take up the mantle of Seneschal after having his/her will and resolve tested numerous times.
Dark Souls is similar in a lot of ways. The First Flame sustains the world as we know it. And ironically enough, that world was forged when the gods defeated the Everlasting Dragons. But anyway, over time, the First Flame begins to fade and darkness creeps into the world. The "Chosen Undead" is the figure that must overcome all of this, defeat the guardians of old, and re-kindle the First Flame.... only for the cycle to eventually loop back around.
In Dark Souls, the final boss is Gwyn, the being who first linked the Flame. The Chosen Undead defeats what remains of him and finally sacrifices themselves to re-kindle the Flame to kick off the next cycle. It should be noted that the lore tells us we are not the only Chosen Undead. Across different worlds and times, multitudes of other Chosen Undead are playing out these very same events.
Obviously, I am stretching things to draw similarities and there are a ton of differences in the lore, but I can't help but draw the parallels. One of the main differences is that in Dark Souls, it's very obvious that the world is dying a slow death. This isn't really the case in DD. Outside of Grigori, nobody in the world really seems to know much about the true nature of the Arisen and the Dragon. Much of it is revealed in the post-game when the player reaches the Seneschal.
I've taken a lot of liberties with this post, because I'm not a lore expert in either of these games. Nowhere near the level of some people out there. So I'm curious if this holds any water or if I'm way off.