You can design a demon in Skyforge, but it won’t be added to the game apparently.
However, you can still design them and win 20,000 Argents. You can submit your ideas to the #contests channel in the official Discord for PC, PS4, and Xbox. Of course, there are rules which must be followed. You’ll have to create an enemy and describe the characteristics, skills, in addition to a sketch of some kind. Note here that you won’t need to draw a full picture of the demon.
Judging will be based on originality, details of its description, the explanation of the abilities, and the appearance. You’ll have until September 20 at 2:59p PDT to submit your ideas. The team will accept up to three entries from a single player.
Note that any of your entries cannot violate game rules, community rules, or the all important user agreement (and the laws of your country). Results of this contest will be announced a few days after the submission deadline ends.
