Mark

Mark is a unique status effect. While a mark does nothing on its own, there are many hero and enemy skills that deal extra damage to a marked target. No character or monster can resist the mark status condition, although the marking skill itself may still miss.

List of enemies with mark abilities

 * Cultist Acolyte's Stressful Incantation (Veteran and Champion levels only)
 * Webber's Web
 * Collector's Life Steal
 * Swine Drummer's Drums of Debilitation
 * Large Carrion Eater's Weaken Prey
 * Fungal Artillery's Mark Prey
 * Wilbur's End This One/End These Two

List of enemies that deal bonus damage to marked heroes

 * Cultist Brawler's Rend for The Old God (+50% Damage)
 * Webber's Bite (+100% Damage)
 * Spitter's Spit/Bite (+100% Damage)
 * Bone Arbalist's Quarrel (+25% for Apprentice, +50% for Veteran and Champion levels)
 * Carrion Eater's Munch (+100% Damage)
 * Large Carrion Eater's Tentacle Devour (+100% Damage)
 * Swine Prince's Obliterate Body/Obliterate Masses (+100% Damage)
 * Fungal Scratcher's Rend The Marked (+50% Damage)

Offensive
Mark skills by themselves do very little and require a both a marker and second attacker to deal damage to make the mark worthwhile. Marks perform well against certain types of enemies:
 * High health enemies that require multiple focused attacks to kill, like large monsters and certain bosses.
 * Enemies with frustratingly high amounts of PROT, like Pelagic Guardians and Ghouls; or DODGE, like Spitters and Rabid Gnashers.
 * Enemies out of reach of the marking hero.

The Arbalest, Bounty Hunter, and Houndmaster all have skills that, at the highest level, roughly double their damage against a marked enemy. This means that a mark effectively pays for itself in terms of damage, and becomes only more efficient with subsequent attacks. Of particular note is the Arbalest's Sniper Shot; its ridiculously high buff to CRIT, on top of the hero's base CRIT (the highest in the game), transforms the Arbalest's otherwise middling attack into a formidable source of backline damage.

Since the mark must be applied first, understanding speed is crucial when building a mark party. It is recommended to pair fast mark-appliers with heroes who can take advantage of marks. One popular and reasonably effective party is the "mark party" comprising the three damage dealers above (Arbalest, Bounty Master, and Houndmaster) along with the Occultist, in some order—each hero can apply a mark, and most can deal bonus damage to marked enemies.

That said, not all marks are created equal. Consider, for example, the Arbalest's Sniper's Mark versus the Occultist's Vulnerability Hex. Both skills appear similar, applying a mark and a debuff to the targeted enemy's DODGE. However, the Occultist is a much faster hero—with, all else being equal, an ~80% chance to act before the Arbalest—and thus a better marking hero. On the other hand, the Arbalest would much rather spend her turn using Sniper Shot. Additionally, since the Occultist is not a strong general damage dealer, Vulnerability Hex serves as a way for him to indirectly damage enemies, especially in the back rank.

A similar comparison can be made between the two PROT-debuffing marks, the Bounty Hunter's Mark for Death and the Houndmaster's Target Whistle—both are superficially similar but serve different roles. Target Whistle has an absurdly high debuff chance (170% at the highest level) lasting four turns instead of the usual three, not to mention a higher amount of PROT reduction. Even ignoring the mark, it is one of the most efficient ways to neutralize PROT in the entire game. Mark for Death, unfortunately is lackluster as a PROT debuff—it has an average debuff chance, requiring a trinket (which Bounty Hunter lacks) to be reliable. Its main benefit is that it can reach rank 4, allowing the Bounty Hunter to indirectly damage the back rank via a teammate.

Aside from the big three, the other mark-boosted skills, while not boasting as high a synergy, nonetheless are useful in their own right. Grave Robbers and Highwaymen tend to equip Thrown Dagger and Pistol Shot, respectively, in order to hit rank 4, while the Shieldbreaker's Captivate is comparable in damage to her bread-and-butter ability, Pierce. In these cases a marking hero can view a mark as simply an additional source of damage, especially if they themselves cannot reach rank 4.

One final caveat about the Arbalest is her slow speed. This video proposes an Occultist, Jester, and Arbalest combination: aside from the obvious synergy between Occultist and Arbalest, the Jester's Battle Ballad solves the party's speed issues, while also boosting the Arbalest's aforementioned CRIT.

As with most things in Darkest Dungeon, the effectiveness of marks is ultimately situational. A mark should always be weighed against another skill—outside of simply attacking the enemy, both the Bounty Hunter and Occultist, for example, have powerful backline stuns that can deny the enemy a chance to act. Also note that since marks have a duration of three rounds (or two in the case of Come Hither), it is relatively more difficult to make use of marks against bosses with multiple actions per round.

Defensive
Self-marking can be used as a defensive strategy—certain monsters, like the Fungal Scratcher, deal bonus damage to marked characters and many will prioritize attacking them, although the exact mechanics of AI targeting are somewhat opaque and highly dependent on region. Since most self-marks, like the Leper's Withstand, come with an additional defensive benefit, they can be used to draw the attention of enemies while mitigating, to some extent, the extra damage. Typical Riposte strategies also tend to involve self-marking, though this is mostly a secondary effect as opposed to self-marking expressly in order to draw enemy aggro.

Overall, because Darkest Dungeon is a game that favours quickly killing priority targets in order to deny enemy actions, as opposed to tanking and healing, defensive self-marking has few highly effective use cases and should be seen mostly as an added, situational effect instead of a primary strategy in and of itself.

Dealing with being marked
Many enemy combinations—Wilbur and the Swine Prince, as a notorious example—use marks highly effectively and when facing them a marked hero is often only an unlucky crit or two away from Death's Door. The threat of increased, often doubled, damage is compounded by the enemy's tendency to focus all their attacks on a single marked party member. Aside from stunning and killing mark-appliers so that they don't get to act to begin with, there are number of other ways of dealing with a marked hero:
 * Guarding is likely the most general and reliable solution.
 * The Arbalest's Rallying Flare clears all marks from her teammates. In particular, this trivializes a large part of the Swine Prince fight. (Note, however, that Rallying Flare does not clear a mark from herself.)
 * Transferring the mark with the Flagellant's Suffer.
 * Granting mark immunity with the Shieldbreaker's camping skill, Sandstorm.

Should none of the above be available, then regular tactics and game knowledge must suffice. Stunning or outright killing mark users can reduce the incoming damage, as can buffing the marked hero's resistances. A number of mark users are also highly position dependent—Spitters and Bone Arbalists, for example, do much less damage in the front ranks, while Fungal Scratchers and Cultist Brawlers can't use their mark-damage abilities from the rear. Forcing these enemies out of position, or failing that, moving a hero out of their reach, can be an effective strategy.