This is Bowser Boss Map with only one command block.
In order to summon bowser you have to build a special altar (shown in the video), place an ArmorStand in the middle of it and throw a fire charge and a nether star.
Remember to be prepared for the fight because Bowser is really evil!
- Command length: 32’700 characters
- Total command block space: 32’767 characters
- Time spent to make this command: about 13 hours