Material Creation

Material Creation (物質創造) is an Intrinsic Skill inherent to the Demon race. Rimuru had likely also gained it through the "Food Chain" of his Ultimate Skill Uriel. There may also be other means of acquiring it as it is seen being used by Velgrynd as well,

Effects
It allows the user to create matter by expending magicules. It is usually used to create weapons or garments, the grade of which depends on how much magicules are used.

Rimuru, upon seeing his naked body after his evolution into an Ultimate Slime, had used the skill to create some clothing which were revealed to be Mythical-grade. Velgrynd had also used it to create her "Blue Dragon Sword" which was also Mythical-grade. However, it was vastly inferior to Rimuru's katana which had been passionately forged by Kurobe, shattering upon impact with it as her sword was merely made of condensed magicules, showing that equipment made using this skill shouldn't be relied on too much.