"Enemies killed by doom propagate the effect around them."
Synergies
The Magnetic Grenade can be used to bring enemies closer together, maximizing the value provided by each triggering of the doom effect.
Since the tombstones created by the doom effect are projectiles, they are affected by mutations Point Blank and Networking.
If there are no enemies near the player after an enemy has been killed by the third attack, the mutation Tranquility will take effect and buff the resulting tombstones.
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.
The tombstones produced by the third hit of this weapon's combo are considered ranged attacks and are therefore affected by ranged mutations such as Point Blank.
The Tombstones created by the Doom effect target enemies that are in hiding, such as Scorpions, revealing their location to the player, but not damaging or making them emerge.
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.