Sir Malik explains they were working with the wizard Benele when the zombies attacked. They lost track of her and now the cultists are desecrating our dead. It's considered an affront to Tu'whacca. Offer to help and he'll explain that the cultists raise the dead soldiers to zombies.
Using this sacred emblem, one can sanctify the corpses and that will prevent them to be raised as undead. He hands it over to the Vestige, then head over to the ruins at Merovec's Folly.
Search the ruins for five dead soldiers and use the emblem to sanctify the corpses. When all are done return to Sir Malik. He will be at the Lion Guard Redoubt now.
Head back to the Redoubt and find Sir Malik to let him know the task is done. He is revealed the Vestige has completed his business.