If you pay attention while you fight the various enemies across the world, there are two versions of the Crucible Knights. There are those who wield the sword and shield and others that wield spears. The spear-wielding knights have a different armor than their sword and shield brethren. Here is how you get the Crucible Tree armor set in Elden Ring.
To obtain the Crucible Tree armor set, you first need to make your way to Deeproot Depths. To reach this location, you first need to defeat the Valient Gargoyle boss. This boss is located in the Nokron Aqueduct dungeon in the northern section of Nokron, Eternal City. Be careful during this fight since you will actually be fighting two gargoyles at the same time. After the fight is over, head over to the waterfall in the boss room and interact with the stone coffin. You will get a cutscene from here where the coffin transports you to Deeproot Depths.
Once you reach Deeproot Depths, make your way across the large gap using the tree roots. Go through the area to the northwestern corner. Be careful to avoid the flying ants and the headless soldiers along the way. In the northwest corner, you will find a large tree stump that you can go inside. The tree stump is guarded by a Crucible Knight boss. You can either fight the boss or get around them using your horse. Open the chest inside the tree stump to obtain the armor set.