The dark apostle is one of the cheaper Chaos Space Marine HQs and packs a decent punch as an analogue to the loyalist's chaplains.
The obvious use for the dark apostle is to place them in a large squad (e.g. cultists) to grant them fearless and hatred. The improved Ld that he also inspires is useful. The ability to also re-roll results on the Chaos Boon table is great: particularly if there's another character about to benefit from.
Without upgrades, he's handy. But with a few tweaks, he could readily become a serious and potent threat. Let's have a look at some possibilities.
Dark Apostle, Mark of Khorne, The Murder Sword, Veterans of the Long War (150 points)
An unsubtle attempt to rack-up as many rolls on the Chaos Boon table as possible. Place in a large squad and go hunting for enemy characters as quickly as possible. Use transports to get there faster! Replace the Murder Sword with the Axe of Blind Fury to taste. Perhaps add a Dimensional Key if there are lots of deep-strikers in the army.
Dark Apostle, Mark of Slaanesh, Gift of Mutation, Veterans of the Long War (130 points)
Taking advantage of the Power Maul, the Mark of Slaanesh should strike early in combat. Roll with some Noise Marines perhaps, or just in a regular squad and a rhino. A cheap upgrade set to take advantage of the basics.