A character falls in battle, and is buried by his comrades (or his enemies, if they consider him worthy). But for whatever reason, the grave isn't marked with a traditional tombstone, but rather with the slain character's weapon. Maybe circumstances didn't allow a regular marker, maybe it's just traditional (for the slain or the ones burying him), or maybe someone thought it would just be a poignant gesture. Weapon Tombstones are often removed by the fallen's descendants or successors for actual use at a later date, especially if they're Ancestral Weapons or otherwise unique.