Harry Potter sat in the dark workroom at the back of Olivander's shop. He had been working on this project all summer. After the Sirius Black incident (despite the fact that he was innocent) where Harry found himself at the mercy of another wizard, Harry decided to make himself a new weapon, one that wizards had never seen.
He had considered multiple weapon types, swords, axes, spears, guns, grenades, brass knuckles, and even dangerous science fiction weapons like laser guns, plasma missiles, disrupter cannons, proton torpedoes, and the like. Unfortunately, Harry wanted something both dangerous to wizards, unexpected (which ruled out most medieval weapons) easily concealed, (which meant no rocket or torpedo launchers) and precise, (which meant no grenades) and capable of going without reloading or recharging, which ruled out guns, bows, and pretty much everything else he could think of.
He spent a few weeks deciding on what he wanted to make. Then he got to work. He paid Olivander to use the back workroom to create his weapon; after all he needed it to have a magical core to work at Hogwarts. He spent thousands of galleons on materials to practice on, and more than a few of those practice runs blew up in his face.
After a few hundred tries, he managed to get a basic prototype nearly functional.
With than done, he really got to work. He burned runes on the inside and outside of the shaft at precise angles. His dragon heart string phoenix feather core was encased in the center of the shaft. The upper end of the weapon was enchanted to never melt, and to contain the incredibly hot substance it would be holding.
He linked the weapon to his magical core, so only he could use the more active abilities that he put within the weapon's capabilities. Runes for summoning, so that he could retrieve it wandlessly, flying spells for so he could use it to escape if need be, adjusting the size of the blade with just a thought, only he could use those abilities.
Finally after nearly two months of work,
Tens of practice runs,
Hundreds of parchment drawings,
Thousands of galleons,
Millions of dreams,
Harry was ready to test his completed weapon.
Harry pressed a button on the shaft of his weapon and a hissing buzz filled the work room along with a green glow.
Harry looked down at his weapon and smiled, his light saber was complete.