The Unity Staff

Find the Weapon Hilt: Head to the Chamber of Lies. You're looking for a sealed rune door. It's located to the right of the exit from the room where you fought the Spider enemy. Go through that door, and you'll find the first piece.

Find the Staff's Head: Still in the Chamber of Lies, make your way to the large chasm in the middle of the room. There's a locked gate on the Northern Edge of this chasm. Now, this is the tricky part: the lever to open that gate is on the Southern Edge of the same room. It's kinda hidden on the wall near the cliff's edge, close to where you fought the Tyrant, Golems, and Dimensional Fire Beast, and where you found a Checkpoint Crystal. Pull that lever, and then hustle back to the gate on the North side. Inside, you'll face a Fire Golem, and after you deal with it and a jump across a pit, you'll find a chest containing the Staff's Head.

Assemble the Unity Staff: Remember the area where you fought those two forgers on platforms across a gap? Go back there. You'll recall a locked gate on the left-hand platform. Now, open your inventory and equip the Staff's Head as Harkyn's weapon. Go up to that locked gate and use a melee attack on it. Boom! It'll open. Inside, you'll find a chest with the actual Unity Staff weapon.

Be Ready for a Fight: As soon as you loot the Unity Staff, a Spirit enemy will spawn on the small balcony platform just outside the room. Make sure you're prepared to fight it as you leave.