Elden Ring is full of optional bosses that you can easily miss in your journey through the Lands Between. The hardest to reach and to defeat is almost certainly Malenia, but Dragonlord Placidusax is also pretty tough to find and even tougher to defeat. According to Steam achievements, more than 60 percent of players fail to defeat Placidusax. Most probably can’t even find him.
Placidusax is so out of the way, both physically and narratively, that it naturally begs a question finally posed by Reddit user grungabunga: "Lore-wise, what's the point of killing this dude?"
To recap for those who don’t live in the Elden Ring wiki, Dragonlord Placidusax was the very first Elden Lord. Chosen by an outer god to rule long before the Erdtree and the reign of Queen Marika, Placidusax fell after his god fled the Lands Between. This led to an eventual war with the Golden Order--a war which the dragons lost. In defeat, Placidusax returned to Farum Azula and secreted himself away in a vortex outside of time to patiently await the return of the dragons’ outer god.
So you could say that killing Placidusax is a kind of preemptive strike. Placidusax would surely attack the Golden Order again after his god returned, so taking him out of the picture now meant that’s not a problem for the future.
There’s another thing to consider. For those who regret taking up the Frenzied Flame from the Three Fingers and want to be rid of it, fighting Placidusax becomes a requirement. Only after defeating Placidusax and using Miquella’s Needle at Farum Azula can the Frenzied Flame be cleansed. We know that Miquella’s Needle can block interference from Elden Ring’s gods, but why it can only be used at Farum Azule remains a mystery.
Of course, there might be a much simpler reason why the Tarnished fight Placidusax. "Cuz we’ve been transported to a spot beyond time, and we asked him if he knew a way out, his answer was to try and kill us."