I couldn’t verify a solid, specific answer for what Mugm’s sword does in Wisp SMP from the available information, and the search results I found only show general Wisp SMP/Mugm mentions rather than a clear description of the item’s effect.

What I can say

  • It may be a custom or meme weapon tied to Mugm’s Wisp SMP persona rather than a standard vanilla item, but I can’t confirm its exact ability from the sources I found.
  • The most relevant result I found was a general Wisp SMP-related post that references Mugm, but it does not explain the sword’s mechanics.

Best match from available info

The safest answer is: I can’t reliably tell what Mugm’s sword does from the sources available here.

TL;DR

No confirmed effect is visible in the sources I could check, so any specific claim about the sword would be guesswork.