Well I can probably still complete the game with my +15 katana. Wish I knew you could unaggro NPCs but oh well only lost like half of my smithing capacity. Oh to make it worse I farmed 9 large taitanite to get a +10 spear... After finding out what the Dragon slayer spear is, it probably wouldn't have helped me much as my faith is still low and I don't think I have high enough strength. Because the +15 iato has A scaling with dex, I can just keep upping my dex stat to get more damage.
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You can't get jumped on by gargoyles outside when you are talking to an npc in undertale.