Gnomes: Find all 4 to get the GnoMAD Scythes (legendary weapon)

Gnome #1:

Area: City of the Dead Fourth Floor
Instructions:

At the northwest balcony, look for a hidden passage. Youll find it behind a turning ledge that you can grab onto.
Open the door with the switch first, then turn around and jump onto the post, then jump to the wall and wall run left. Return to the hand holds, and use Deaths split soul to get off the nearby pressure plate, exposing a secret room.

Gnome #2:

Area: Ivory Citadel Second Floor
Instructions:
After purifying the pool on top of the westernmost tower, return to the northern passage that was previously covered in ooze on the second floor.
To get back to the towers top floor, follow the aquaduct and climb the wall, but dont leave through the door. Instead, drop down to the balcony below and climb back up to the top of the tower.

Gnome #3:

Area: The Weeping Crag Third Floor
Instructions:
To find a secret area, youll run into a pressure plate / rising platform puzzle in the third floor of The Weeping Crag.
Past the platform is a portal. Open it, then turn east and use the stairs, turning north and looking up at the ceiling for another portal to open. Activate it, then return to the previous portal the two portals should now be linked, and youll fall out of the ceiling portal into a secret room.
Inside the treasury, dive around the northern waters for a secret underwater tunnel.

** Alt method before obtaining Voidwalker please review second video.

Gnome #4:

Area: The Black Stone First Floor (Present)
Instructions:
Wait until after youve unlocked the door in the past. When you return to the Great Foyer, check the northern ridge for hand holds. Descend the ledges, and return to the present through the portal.
Now in the present, climb back up using the hand holds to the north and go through the now open westernmost door. The gnome is located deeper down the hall.

The GnoMAD Scythes are sent to your offline drop box at a Serpent Tome, as soon as you claim them, the achievement pops.