Gedächtnis Essers are fleshy colored with a dark maw and multiple tentacles.
Gedächtnis Essers use four tentacles to burrow into their victim's heads to steal their memories which can be used to take over their identities. It takes considerable effort for them to process these stolen memories. Furthermore, it seems the memories extracted from a Grimm will leave them in a traumatic state given the violence they often inflict upon Wesen. They also are terrified of Grimms.
It is not known how this Wesen removes the memories of its victims, only that it can. I surmise it has something to do with the electrical charges in the brain. After an attack the victims are usually diagnosed with advanced dementia.
From interviewing some of these unfortunate souls, I can see why. This is one of the most heinous Wesen I have ever encountered.
They rob the victims of their very lives by stealing their past and leaving them void of any human connection, devastating to their loved ones, and leaving the victim unable to identify the attack or the attacker. Justice within the confines of the law is therefore impossible.
I witnessed this Wesen attack a woman -- four of his tentacles pierced the back of her skull.
Henry Slocombe's skull after the attack of the Gedächtnis Esser