Not every game that has soulslike elements is Dark Souls or by FromSoftware, and these four games show just that.
Boss Soul weapons are weapons which can be crafted from the souls that are dropped by some bosses throughout the game. There are multiple weapons which can be constructed from the souls (usually two).
Your next stop on the quest for the Lord Souls is the New Londo Ruins. To get to the ruins, go down the stairs near the bonfire at the Firelink shrine past the Fire Keeper and go down the elevator. If ...