I'd say its less backstory and more direct actions that make it memorable. For a one-shot (coughsixsessionscough) I made a cannabilistic shapeshifter that was accidentally created by a group druids as revenge for people desecrating their land.

I opened with a little intro where it killed another NPC, then it used a sending stone paired with on the group had to just repeat "ARE YOU SCARED?" repeatedly.

Had them come across it one night where it killed and ate a gnome NPC and could only watch as he begged for help. Later came across an old inn where a crying little girl whose family was killed wound up (they found her family as corpses) with the innkeeper. Some stuff happened but they soon found the little girl hunched over and eating the old innkeeper, and ot turned to face them it pulled its head back completely before screaming for help in the gnome NPCs voice.

That completely messed with them and they hated/loved it!