You don't need to fight Elden Ring's Placidusax boss to complete your journey in the Lands Between, but if you're determined to take on every boss, this dragon is very easy to miss. It's also required for one of the achievements, so you'll need to take him down if you're a completionist.
You won't be able to reach this boss until you get to one of the last areas in Elden Ring, so if you're just starting on your journey, you won't have to worry about this for a while yet. I'll do my best to keep spoilers to a minimum, but be aware that late-game place names will be revealed below. Here's how to get to the Elden Ring Dragonlord Placidusax location so you can beat this secret boss.
Elden Ring Dragonlord Placidusax location
When you find Placidusax, you'll wish you were the first person to discover it. The area is extremely hidden, and you won't be able to access it until you've worked your way through to the last Site of Grace in Crumbling Farum Azula. This area's part of the story, and you'll go there once you're done with the Mountaintops of the Giants.
If you already have the area fully unlocked, it's a fairly simple—but extremely cool—process to get to Placidusax: From the Beside the Greatbridge Site of Grace, head back down the lift instead of taking the normal path to the final boss of the area. At the bottom, take the stairs to your right, run past the enemy directly ahead of you, then veer left and make your way outside. There are a couple more enemies to deal with here, but you can run past them without too much trouble. Though you'll want to be quick with this next step to get out of their aggro range.
Once outside, head to the edge of the cliff right in front of the building you've just left and look over the side to find a rock platform where you can drop down. From here use the other rock platforms to make your way steadily down towards the big slab of rock at the end. Once here, you'll see arch-shaped indents, but only one of these doesn't contain a skeleton—this is the one you're after. Once you approach it, you'll get a prompt to "Lie down".
I won't spoil the cutscene that follows, so wait until it finishes (or skip it if you're an absolute monster) and you'll find you've been teleported to the Dragonlord Placidusax's boss arena. Don't worry; he won't aggro until you get closer to him so you can admire him from afar before taking him on.