The third Dark Aether Rift is one of the new additions to the Zombies mode in MW3 and one of the relics needed to open it is the Improved Imaginary Friend Drawing. This is how you can get and upgrade it in MW3 Zombies.
Where to find the imaginary friend drawing in MW3 Zombies (location)
The Imaginary Friend Drawing Relic can be found under a second-floor bed in a mansion located at coordinates I7 on the map above, east of Shahin Manor. By looking under the bed, you will not receive a message to interact with it naturally.
To receive the interaction message, you will need the Perception of Death perk. But even without that buff, pressing the interact button will cause the Dark Aether Entity to speak, after which a reward rift with the Imaginary Friend Drawing will appear.
How to update the imaginary friend drawing in MW3 Zombies
After getting the imaginary friend drawing, head to the previous location at coordinates F5 on the map and you will find a pink/purple triangle. Stand in the triangle and shoot the runes at the buildings around you (in any order). Then, go to the top of the bridge-like structure near the triangle and you will have a chance to perform a summoning message.
Initiating the summon will cause zombies to appear within a purple radius. Eliminate them quickly. A special zombie will appear later; Killing the special zombie will reward you with the improved imaginary friend drawing through a reward rift, which will be vital to opening the Third Dark Aether Rift.
For more information on Call of Duty, check out how to get the Specialist perk in Warzone, here on MyFullGames.