It’s listed at Sketch – Knight’s Sword under books in his shop inventory. You can likely get it down by 50 groschen or so with some haggling. You’ll generally find Osina next to the forge ...

And having been saved, all that was left to him was to wage war. The Fire Knight Queelign uses his sword to attack and burn enemies. It can also cast Fire Spells like other Fire Knights.

Moonrithyll's Knight Sword can be obtained by defeating Moonrithyll, Carian Knight, an NPC phantom found in Castle Ensis. Check out the map below for the exact location.