The Masamune Bug
I just played a game of sealed in which I had the Masamune attached to a Gaelicat, also with the Monk’s fist equipped, making it a 4/3 flyer with vigilance, and first strike on my turn. The card text also reads “As long as creature is attacking, it must be blocked if able. The opponent had a PuPu UFO untapped in defense. I declared attacks and they were not forced to block. On the next turn, the PuPu was able to attack and did damage, when it should have already been dead. I am unclear why the opponent’s creature did not have to defend.
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Rezzahan#77802
commented
Was your Gaelicat blocked by another creature? If so, the PuPu UFO was not required to block as the Gaelicat got blocked already, fulfilling the Masamune's requirement.