The 5-6 player expansion pack adds 2 new characters, 3 new locations, new spells/counterspells/companions, and some new mechanics into the game (in particular, freeze and stun status effects, which is why it comes with status effect tokens). It does let you expand the game for up to 6 people, but the 2 new characters and hex tiles can still be used even instead of characters and hex tiles from the base game even if you’re playing with fewer people.

The Freeze status effect prevents an enemy from using the Travel action on their next turn

The Stun status effect reduces the number of resource dice an enemy gets to roll on their next turn.

This is an expansion – the main game of Casting Shadows is required to play this