Dragonlord Placidusax is one one the coolest bossfights in Elden Ring so i will tell you how to get to it. But before we get into it, i want to tell you a fun fact. After you beat Godskin Duo and go down the main path, you can see the remains of the DragonLord imbedded to the wall. It makes sense because when you get to the bossfight you go back in time.
DragonLord Placidusax is located in crumbling farum azula which is accessible when you light the forge of the giants. The easiest way to get there is to travel to the grace called "beside the great bridge" and go down the elevator. Then run past the room with 5 Azula Beastman and run to the cliff. You might see a lot of messages that tell you to jump down.
If there are not any messages near the cliff then just follow waht is say. Walk around the cliff until you find a suspicious looking crack. Jump down and there will be more platform that you can jump on. Keep doing that until you reach a long gray platform with skelotens of Azula Beastman. You will need to stand on the section of the platform without a skeloten and interact with it
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