Smashing your first Soulstone in The First Berserker: Khazan is a bit confusing—a little message pops up saying "You've destroyed a mysterious energy-emitting ore" but nothing else happens. You can find these red glowing crystals scattered throughout each of Khazan's missions, and while you might have assumed they impact the ending as mysterious objects often do, they actually provide bonuses.
Destroying enough Soulstones and then talking to the right character lets you gain some buffs that'll help you out in combat.