Here you go, here's some info about getting the afore mentioned weapon.
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When you S-Rank Seth-Rah, it will say you got the Longinus, as you have seen. The game(s) you start after S-Ranking Seth-Rah will have Longinus in the place of the Rosier in Heaven's Gate, as Sturm said. However, if you play through and don't get the S-rank the 2nd, 3rd, (etc.) time, It will go back to being Rosier.