Marked is a condition.
- You take a −2 penalty to attack rolls for any attack that doesn't target the creature that marked you.
- You can be subjected only to one mark at a time. Newer marks supersede older ones.[RC:232]
- A mark ends when its creator dies or is unconscious.[RC:232]
Defenders like fighters, paladins, wardens, etc. usually have a class-specific mark and a class-specific way of applying said mark and punishing violations of said mark(s). Marking opponents is one of the defenders' core features.
Monsters of the soldier role are also able to mark players and act as the monster equivalent to defenders. Just like defenders many soldier role monsters are able to take special actions and apply special conditions and punishments to those they have marked.
Marks are mutually exclusive, meaning that unless it is specifically stated otherwise in a power, applying a mark to a target erases the previous mark(s). I.e. the vast majority of the time it is impossible for two different characters to mark the same target and it is impossible for a single character to mark a single foe with more than one mark.
In its most basic form, being marked means that you have a penalty to attack rolls against everyone, except the one who has marked you. For more advanced marks, violating the mark, by making attacks which do not include the one who has marked you, will trigger some sort of punishment; usually in the form of attacks or outright automatic damage.
Marks cannot be applied by just anyone — you must have a power, class feature, item, or some other effect that allows you to mark creatures. The method for marking targets varies from class to class. A battlemind can mark a creature with a minor action, while a fighter marks with every attack, even on a miss. A rogue, however, normally has no mechanism for applying marks. A ranger can mark a target with the marked for death power, but this is a rare occurrence.
- The PHB originally contained only the -2 penalty to attack rolls. Errata added two additional clauses to ensure marks aren't permanent.