Marnassier was a Mauvais Dentes (Sabertooth Tiger like creature), he was hired by Eric Renard to hunt down Nick's key.
Renard arrived in America, hiding on a cargo container, he was found by several guards, but he simply killed and ate them. Before leaving and hiding in Portland trying to track Nick Burkhart, so he could kill him and take his key.
He lured the two FBI agent who had been assigned to investigate his murders to the old saw mill, where he captured, tortured, mutilator and then killed them, but before he did, he forced onto to call Nick, so Nick would come to find him. Nick arrived, however Marnassier disguised him self, as one of the corpse. And ambushed Nick from the back.
But Nick's Grimm proved powerful enough, to roughly match Marnassier's own superior skills. The two fought for a long time, Marnassier constantly tricked Nick, trying to exploit his weaknesses. He was just getting the upper hand, when Kelly Burkhardt, Nick's mother attacked him from behind. He managed to hold the two Grimms off for a few momments, but was quickly out matched by them. Misjudging his opponent, he lunged at Nick, but Kelly caught him with a knife to the throat. All he was willing to admit, was he wasn't expecting a second Grimm. He threw Kelly of, but seeing the attack coming she plunged her knife into his neck, killing him moments later.