A Demoknight is a Demoman that uses the Eyelander with the Chargin' Targe. But mainly all a demoman needs to become a demoknight is the Chargin' Targe. In a game people can call for a Team Deathmatch consisting of Demoknights only. Although you'll still have respawns unless in arena it can be entertaining.
We should do Demoknights! - On Team Deathmatch
Wow; Another demoknight - On amount of Demoknights