Grim Demeanour mechanics

Hey fellow Kyn, I'm new to 40k and have started with Votann and have loved the few 500pt games I've played so far. Currently running my Kahl with Wayfarers Grace (cos he died before doing anything 2 games in a row 🤣), but also because I don't fully understand how Grim Demeanour works 😅

Could someone please explain it, and with which attached units it works best with? Thank you!