Lots of debate over a guaranteed drop, so I looked at the game's internal params. Here's the deal, it has a base 2% chance to drop from the necromancers, but it uses something akin to bonus points with a value of 10, meaning every time it rolls for the drop and doesn't drop, it adds 10% to the drop rate of the next roll. When it successfully drops, this resets back to 2%.
So by the time you kill the fourth necromancer, your drop rate for the lantern on the fifth is already at 42%. Given there are 6 necromancers, it's statistically unlikely that you won't get the drop, but it IS possible.