From the video description:

CORRECTION: One of the shops in the first episode does sell the poison antidote (the one in the cave). Guess I missed it somehow.

On this episode of PC-98 Paradise, I play “Madou Monogatari A-R-S”. A dungeon crawler made by legendary game developer “Compile”, which also has characters that were later used for their puzzle game series “Puyo Puyo”. This one is the sequel to “Madou Monogatari 1-2-3”, and was a PC-98 exclusive. There might also be a surprise bonus near the end of the video…