Dark Souls: The Lore of the Optional Bosses

gamerant.com:

A series known for its obtuse story, Dark Souls doesn't just tell its players what's going on. Instead, it gives them the tools to discover the truth for themselves and fit together the rich backstory of the game's world piece by piece. Through item descriptions and environmental storytelling, and years' worth of research, Dark Souls fans have finally unraveled the mysteries of Lordran.

As is the case with every From Software game, the first point of interest for fans areDark Souls' bosses. While Dark Souls' main bosses have deep connections to the origin of the world and have directly impacted the environments players are walking through, most of the game's optional bosses are viewed more as side characters. That being said, the lore of the optional bosses is still just as compelling.

Dark Souls: The Lore of the Main Bosses

Created when the Witch of Izalith disastrously attempted to create her own First Flame, the Taurus Demon is considered a lesser demon, being one of the weakest of his kind. Being located in the Undead Burg, the Taurus Demon is quite far from its home of Izalith.

Though the exact reason for its appearance in the Undead Burg isn't known, it has been speculated that the Taurus Demon was sent on some mission by the Bed of Chaos to take back the Burg from its human settlers, but has gone hollow in the process. This could explain the appearance of a Black Knight stationed near the Demon's fog gateway, as these warriors fought the Demons years prior at the request of Gwyn.

Created in the same disaster as the Taurus Demon, the Capra Demon can be found in the lower area of the Undead Burg. His presence here could hint that he accompanied the Taurus Demon, or could imply that he was sent to rescue the Taurus Demon