S The Tragedy of M?

One day, when Shuna and Shion are fighting over Rimuru's squishy slime body as usual, Rimuru comes up with the idea to make a cushion in the shape of his slime form. He travels around Tempest to see if his friends know of any good materials he can use, and he learns that a special type of sand found along a lake shore in the forest is exactly what he's looking for. So he sets out to collect some sand... but a monster is waiting for him by the lake!

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S01E27 - Hey! Butts!