Obelisks are a device used to trap groups of hostiles.
Obelisks are dropped by airships. The potential victims are kept contained by Mega Mechs until the laser fence emitted by the Obelisks completely encircles the enemy. Contact with the lasers results in the disintegration of the victim. They appear to be a weaker, smaller variant of the Espheni defense grid.
On their return to Charleston, South Carolina the 2nd Massachusetts Militia Regiment was ambushed by the Espheni as several Airships quickly dropped Obelisks emitting a laser fence into place. At least one Mega Mech sprang out of the ground and opened fire on them. Some of the humans were able to escape, but many including Tom Mason and Daniel Weaver were caught inside the fence. Lyle attempted to help Matt Mason escape but a portion of the laser fence activated in front of him, causing him to run into it and get disintegrated. After being hit by a pressure cannon, a Black Hornet suffered the same fate. (Ghost in the Machine) (Exodus)
In Exodus it is revealed that a suit, created by Dingaan Botha, can withstand the obelisks' lasers for enough time to climb the fence. This is crucial in how the 2nd Mass escape from the Espheni ghetto as it allowed John Pope to climb over the fence and sever the energy tether powering the fence.