Legion of the First Prince

Allied Bloodletters, Horrors of Tzeentch, Plaguebearers and Daemonettes units benefit from the Mark of Chaos battle trait as if they have the SLAVES TO DARKNESS keyword.

At the start of your hero phase, you can pick 1 LEGION OF THE FIRST PRINCE UNDIVEDED unit and then pick 1 of the following Marks of Chaos keywords: KHORNE, TZEENTCH, NURGLE or SLAANESH. That unit has that Mark of Chaos until the start of your next hero phase in addition to the UNDIVIDED Mark of Chaos.