The Sword gravestone is an ancient exploit used for many generations by Kings and Warlords. It allows the optimal use of a unit, as it forbids the knight buried beneath a Sword Gravestone entrance to Heaven, forcing them to come back to earth as the undead and fight their battle once more.
However this second chance at life comes with a price, because if they die a second time the only gates that they can be allowed through are the ones below.