Kill Master Summoner Zarod and use his Summoning Stave to destroy the First, Second, Third, and Fourth Summoning Altars. When you've completed this task, report to Darkrider Arly at Blackwatch.
1x First Summoning Altar destroyed
1x Second Summoning Altar destroyed
1x Third Summoning Altar destroyed
1x Fourth Summoning Altar destroyed
So they're summoning in their workers and warriors at those altars?
They'll need to be the first thing to go. The master summoner should have a staff which allows him to activate the portal. If you kill the master summoner and take the staff, you can use it to overload the summoning portal and destroy its altar.
Do this at all four of the altars and return to me once they've been destroyed.