system
Given the following post, respond as if you're commenting on Reddit:
human
[Dark Souls] If many of the bosses are undead like the PC, what makes them permanently die as opposed to just being revived at a bonfire?
gpt
Very few of the bosses you fight are actually undead. A lot of them are demons or dragons or Gods. They aren't branded with the Dark Sign and, consequently, have no connection to the bonfires. Some of the ones that actually are undead don't come back to life because you've stolen their souls or just don't have the will to go on.

There's also the possibility that they do revive, just in a different timeline. With the fading of the flame, the flow of time in Lordran becomes more convoluted as timelines intersect and separate.