Beneath the full moon.
Not far from Yechen's outpost, a strange worm burrowed out of the ground.
It was a full three meters long, with a round head and wrinkled bumps covering its body, as though it wore a bizarre suit of leather armor. Stranger still, it resembled an earthworm, yet differed in certain ways.
It swayed its head from side to side, then spat a figure out of its mouth.
After emerging, the man warily surveyed his surroundings. Only once he was certain it was safe did he stroke the worm's head.
"Good work, Bruno."
The worm enjoyed his touch, then lay quietly on the ground, awaiting his orders.
Had Yechen been there, he would have found the man somewhat familiar.
He was Matsui Shūji's older brother, Matsui Shūichi.
The worm named Bruno was his summoned beast. Not only could it conceal its presence and track scents, but it also had an alternate dimension inside its body.
Matsui Shūichi had hidden inside that alternate dimension, relying on Bruno's ability to conceal its presence to follow Yechen and his companions undetected.
He had followed Yechen to find an opportunity to avenge his younger brother.
Although he had sometimes warned his brother not to be so brazen, that was still his only younger brother. Yechen had mercilessly killed him with two slashes. How could he, as the older brother, remain unmoved?
After learning that his opponent was Yechen the Flame Demon, he had not acted immediately. Instead, he used his summoned beast's abilities to track him all the way here.
The watching gaze Yechen had occasionally sensed had come from this worm.
As for why he had not reported the information to Iwagakure and had others surround this Konoha team...
The answer was simple. He knew exactly how strong this Konoha