This is the 9612th most frequent Spanish word.
apuñalarlos
"To stab them."
Here 'apuñalarlos' is used to describe a literal physical action, referring to stabbing someone.
Los villanos planearon apuñalarlos mientras dormían.
The villains planned to stab them while they slept.
'Apuñalarlos' here is used metaphorically, meaning betrayal rather than a physical stab.
Debes evitar a esas personas que quieren apuñalarlos por la espalda.
You should avoid those people who want to stab them in the back.
In this usage, 'apuñalarlos' indicates a dramatic storyline twist affecting the characters, not a literal stab.