"Enemies killed by doom propagate the effect around them."
Notes
Currently, the tombstones that are placed on the ground when killing an enemy using the third hit or with the doom effect do nothing.
Similarly to the Alchemic Carbine, the Tombstone cycles between several visually different attack animations that feature various different kinds of tombstones. This difference is cosmetic.
The doom effect is affected by the affixes applied to the tombstone the player is carrying.
The third hit of the combo bypasses shields, much like whip-type weapons.
Trivia
Tombstone is featured in the update banner for v2.3, aka the Whack-a-Mole Update.
At one point in the alpha build of v2.3, placed tombstones were originally intended to apply the doom effect when destroyed instead of on the weapon's third attack in a combo.