Defend your necropolis against hostile attacks. Choose a unit and attack it. During the 'tactical break' you can cast spells to keep the enemy away. The more captains you kill, the more skill points you earn, that you can use to recruit troops for your own army.
Click on a unit to select it. Then choose the attack: Press A to select all the troops, Q to just attack the adjoining unit, R to remove the selection and E to make your troops return to their original position. Use 1-4 to select a spell.