L'accetta

Hello everyone ive been playing undertale for a while now but theres something i dont understand, basically my friend that played undertale a lot said that i should be very careful in the ruins because i might encounter something or someone named "gaster con l'accetta" (its in italian, the translation Is gaster with the axe), but i havent been able to find any trace of this character, and since my friend's been explaining It like its a really scary event, id like to know how to avoid It or atleast be ready when it happens, if anyone knows about this or encountered this id appreciate if you could explain to me what they are because i think its creepy.

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