Infinite Sand appears on floors 1 and 2 of Magatsu Inaba within the Midnight Channel. It will try to Enrage the party and also have the Rage Boost passive skill to increase its chances of successfully affecting the party. After being defeated, it may drop Nightmare Sand. Selling 4 Nightmare Sand will unlock the Fuuma Kotaro daggers for Yosuke Hanamura.
In Golden, they appear in the same floors listed above. It may drop Aurora Quartz for trade at Shiroku Pub.