Rygar is an adventure game for the NES, and is a single player experience. It was published by Tecmo Inc..
It should be easy to find Rygar for the NES available for purchase. In good condition, the game is worth about $5.00. Rygar was released in 1987.
I think this is a glitch rather than a code... If you If you stand on the left side of the door while turned left during the fight with Belzar, he'll shoot a laser at you. You'll be pushed through the door into a crazy scene. This will kill you instantly - press start, and you'll be at Ligar.