Well, gentlemen. I assume this means I have the job.
-- Grimmson, when Newt Scamander refused to hunt down Credence Barebone (CG)
Grimmson was a rough and obnoxious “Beast Hunter” who offered his services to the Ministry of Magic in 1927 to hunt down American Obscurial, Credence Barebone (CG). He was chosen for the job after Magizoologist Newt Scamander refused to become an Auror and hunt down Credence, for whom he felt only pity.
Grimmson hunted Credence through the streets of Paris as he walked along talking with his friend Nagini, and finally confronting him at the home of his one-time nurse, Irma Dugard. Grimmson performed a Killing Curse that seemed to accidentally strike Irma, infuriating Credence, who was seeking answers about his real family. Credence unleashed the fury of the Dark energy inside him, destroying the room where Irma had lived but having no effect on Grimmson, who saved himself with a bullet-proof Shield Charm. Nagini also escaped, since Credence did not attack her.
Grimmson then met with his true master, Gellert Grindelwald, telling him about Irma’s death, which was no accident at all. Grindelwald seemed to want Irma out of the way so he could control the story of Credence’s origins and adoption, and after that, control the power of Credence “for the greater good” (CG).
The ability to perfectly camouflage himself. Powerful magic skills to shield himself from harm.
Spelled as Grimm, the name may be a reference to the Brothers Grimm, collectors of German folk and fairytales.
Since an Icelandic actor was chosen to play Grimmson in the Crimes of Grindelwald movie, a similar name from that country cannot be ignored. Spelled "Grimsson" the name means "Son of Grimur, a common Icelandic male name through the centuries.
According to "Fantastic Beasts: The Crimes of Grindelwald: Movie Magic" by Jody Revensen, Grimmson's wand was "an old piece of knotted, gnarly wood. Really rough and almost hand-whittled, with whorls in it." "He's also carved mementos of his kills in it."
The same book gives him the first name of Gunnar.