The Mask of Edki is a yellow wooden mask seen in the Me and The Eds episode Eddy of The Mask.
Casimus is seen making a wooden mask in woodshop. He paints it yellow, and plans to give it to Eddy. Eddy reads a flyer about a piggy-bank pinata at The Tiki Hut club/casino (the 1st casino made for teens) where Nazz is forced to work as a sexy dressed waitress and on some nights, a singer. Eddy is just about to make a call to go with Kevin, until he reads a small line that reads "Ages 13+ only". Eddy is annoyed because he is still 12 and Edd just turned 13, and was just invited by Casimus to go for a night on the town. Eddy is so mad he steals the Mask. He goes home with dread. He looks into it and is transformed into a teenager version of himself.
Resemblence to the Mask of Loki
The Mask of Edki is slightly different than the Mask of Loki. First off, it's painted yellow instead of green. Instead of turning its wearer into a green skinned man, they age into 13 (if they already passed 13, 1 age higher than thier regular age).