Eren was almost being too careless after the bear win. And he definitely didn't expect any second threat after the first one. It would be unusual.
But his ears caught it and forced his body to act before his brain did. A twig snapped fifty meters to his left. Then a second of complete stillness that only predators knew how to create. It was almost like nothing was around...
Shadow Panther or some kind of a cat type beast but he imagined it would be that famous hunter with his luck.
Emily had described them once between bites of dinner. Around late 10 early 20 level ambush predators. Fast enough to kill most things before you knew you are being hunted. She did say something like "if you hear one, it's already too late" and then gone back to eating like that was a normal sentence.
To Eren's half-elf senses at Perception 37 after level up, the thing might as well have been wearing a bell.
