Ending

To Link the Fire and Dark Lord are the two alternative endings that both occur in Dark Souls, it is the players choice in which they want continue The Age of Fire, or The Age of Dark.

To Link the Fire
To Link the Fire is one of the endings that occur by defeating Gwyn, Lord of Cinder and going to the Bonfire nearby and Linking the Fire. By doing this ending, you sacrificing your own self, to bring the fire and light to our precious world. It is optional to do this ending. As seen in the cinematic you start off as getting burnt, then more burnt, then completely incinerated and after the credits roll. This is normal, you can still do another playthrough after this. You have achieved an achievement or trophy by doing this ending and succeeded in Kingseeker Frampt's Goal to bring the fire and light. Aswell as Princess Gwynevere's.

Dark Lord
Dark Lord is one of the endings that occur by defeating Gwyn, Lord of Cinder and going through the pathway or door. By doing this ending, you are ending the Age of Fire and bringing a new era, called the Age of Dark. In the cinematic, all primodial serpents bow down before you, calling you their Dark Lord, chosen undead. This suspicionly surprising because Kingseeker Frampt is there, bowing before you. Frampt wanted you to link the flame. But it is unknown if it is a betrayal or not. By begining this era, you are bringing all the Darkness to the world, in which is the opposite of the other ending. You are not sacrficing yourself. You have became a Dark Lord. By doing this you have achieved an achievement or trophy and you succeeded in Darkstalker Kaathe's Goal in bringing the Darkness.