Don't know the exact procedure to decide how Mordenkainen is 18th level in 5e but I did come across a video from Chris Perkins being intereviewed where he states that if he fully stated out the wizard he'd be beyond 20th level (so I assume 20th level). https://youtu.be/FcMtfc7WmnY?t=853 RinVindor (talk) 04:27, 13 May 2021 (UTC)
- The wiki's policy it to follow printed materials. So if 5e says 18th it is 18th. This sadly is not the unique level-based weirdness. Other characters have jumped around levels across editions or leveled up negligently or even switched classes altogether. So the best action here is to shake your head, document what's published, and assume for personal use that Mordenkainen is as overpowered as your home campaign needs him to be. Artyom.pavlov (talk) 04:40, 13 May 2021 (UTC)