To be fair, Satan is not involved in this episode of Grimm, nor is a Weseen — it's just a parasite from Jordan that turns little kids into creatures that seem exceedingly Satan-like, but it does provide this week's Grimm with a bit of balance.
First, lets go with the "sub-plots" of the episode, just for kicks. Renard goes to Austria and hides and gets attacked and hides some more, while Adalind meets a prince of the royal family, who isn't revealed because meeting a royal prince and revealing that prince is obviously just too much for Grimm to take on in a single episode.
The real meat in the episode is of course a boy who freaks out during his own exorcism, knocks a seminary student unconscious and kills an old priest, to his parents' gloom. Apparently this kid has been evidencing Satan-like behavior for a while now, and the exorcism was the parents' last hope. Nick investigates, and he acknowledges it it could be a Wesen, but since neither of the kids' parents seem to be Wesen, it's all very weird.
The requisite Munroe and Rosalee chats reveals the existence of the Graussen, who are basically Damians from The Omen that have appeared throughout history, who the royal family have traditionally murdered throughout history because they end up slaughtering continents full of people or something. Here's where it gets interesting: Apparently, the Graussen is so taboo that Rosalee reports it to the royal family, who sends an assassin to kill the boy; Munroe, when he learns of Rosalee's actions (not to mention the assassin) tells Nick, indicating a fair amount of cultural tension between the couple (although this is Grimm, so we'll see how serious this really gets).
The hospital discharges Damian because they don't know what's wrong with them — because that's what hospitals usually do — the royal family's assassin hides in a closet and waits until someone walks in and reveals the kids' location (thanks, Nick!), and it all ends up in a chase scene in the woods outside Damian's parents' home, where Nick beats the assassin and kids gets so cold that the parasite dies. The kid is fine! The problem was a Satan-like parasite lurks in the Jordan river. So… non-Satanists should probably cancel their Jordan vacation plans, I guess.
Rereading what I wrote, this episode seems pretty banal, but the Renard's European Vacation weirdly had some weight to it despite very little of anything happening, and I respect that Grimm is going outside of its Wesen sphere to present Nick with other mysteries besides fairy tale monsters. It would have been cooler if Satan were actually a player in these stories, but at least the world is bigger than the new Wesen Nick arrests each week.
• Juliette does the heavy lifting in regards to the non-Wesen research this week; again, it's awesome she's being helpful, but that gives less time for Munroe and Rosalee to be helpful, and think we'd all rather watch Nick and Munroe, right?
• Rosalee seems really unlikable in this episode, which, I know she's more or less advocating the murder of a child, but the show doesn't even try to capitalize on the goodwill she's earned so far. She's just, "Sorry, it's the rules."
• Admittedly I would LOVE it if Rosalee took a heel turn.
• So this parasite only affects kids, I guess? Because Munroe and Rosalee seems pretty adamant that Graussen are kids.
• Having started Grimm in the middle of the second season, I haven't gotten to see Nick really own his Grimm-ness much. When he schools the Royal Family's assassin, that's pretty great.
• David Giuntoli looked shockingly pale during some of tonight's scenes. I can't tell whether this was an intended nod to Nick's half-zombie status or just weird production values. If anything, that sums up Grimm for me more than anything else: If it's doing something clever, I don't know if it's intentional or on accident.
• I'm forming this theory that Wu knows all about Wesen, and he's keeping quiet because he patiently waiting for Nick to bring him on Team Grimm. It makes his constant eye-rolling at people after he relates all those weird "facts of the case" far more enjoyable.
• For all my problem with Grimm, I'm really excited about Krampus next weekend.