This Legacy Chest can be found within the Spire of Light dungeon, in the region of Hallowed Marsh. If you’ve never been inside before, you will need to enter the dungeon from the bridge to the north west of it.
To reach the Spire of Light, take the road south east from Monsoon and follow it until you reach the bridge to the dungeon. Upon entering, you will encounter two Golden Minions. These are fairly powerful mobs which are immune to both poison and bleeding, but vulnerable to Ethereal and Decay damage. Aside from their basic attacks, they have one powerful area of effect lightning attack which inflicts significant lightning damage and a debuff of -25% lightning resistance.
With both Golden Minions dead, the first thing you should do is walk down the stairs immediately adjacent to the entrance and pull the lever in the centre of the room, as shown below. Once the lever is pulled, two other mobs will spawn on the stairs; another Golden Minion and a Golden Sepcter. They have the same immunities and resistances as the minions, but with additional skills. Those with cannons are able to fire lightning blasts which can trace your movement for a short period of time. Use physical barriers to block these attacks and retaliate immediately after they have been executed. Cannon specters also fire a lightning area of effect blast which deals damage over time, so be sure to avoid this when executed. Those with blades have only basic attacks, but they are swift and deal significant damage.
When all mobs in this first room have been defeated, make your way up the stairs to a wooden platform at the top. The stairs lead only one way and you cannot progress before pulling another lever, so don’t worry about going the wrong way. When you reach the wooden platform, pull the lever to the right of the mechanism in its centre. Now pull the lever in the centre of the platform.
At this point you can go back down the stairs, two stairways down, until you reach the blue orb that you saw on the way up. You will notice that you can now enter it and interact with it. Before you interact with it, make sure that you are prepared for combat; you will be teleported to another platform where some Golden Minions and Specters will be waiting. Be sure to loot the chest behind the orb before you go. Once you’ve dealt with the mobs, pull the lever behind the blue orb that you teleported to and loot the nearby junk pile. You can now cross the bridge to the north and interact with the orb to teleport again.
Whether you fight the Light Mender or not, once you’ve pulled the lever behind the throne, make your way back downstairs to the entrance, using the orbs to teleport. From the entrance, turn right (south west) and use the door in the corner to progress to another section of the Spire of Light.
Here you will find two more Minions and another blue orb. With the minions dead, continue through the doorway to the south east and use the lever in this room, unlocking the blue orb nearby. Teleport using this orb to enter the upper levels, where you’ll find another Golden Specter. Use the lever in the north east of the balcony to open the iron gate to the final orb below. Using this orb will teleport you back to the main entrance room, where you will encounter one more Golden Specter. After defeating it, you will be able to access the Legacy Chest up the stairs to the north east. You should also interact with the tapestry next to the chest to acquire the Pillar Greathammer.