A Mark of Chaos Ascendant is a pretty big deal. What it should mean is that your players have the attention and favor of the Dark Gods as a chosen champion.
This is not something given out freely, or rashly, and with it comes great power, but also great expectations.
Slap them with potent buffs of about ten points to all combat skills, and give them plus ten to all interaction rolls. Give them Infernal Will and Steel Hearted gifts, Daemonic Names from all four Dark Gods (but only one 1d5 infamy bonus) and all four Marks of Chaos with the effects therein.
Finally, give the party the right to choose some Daemon Weapons, imbued with a Daemon of any allegance, with one handed out per player.
Of course... if the players should actually disappoint the Dark Gods or fail to deliver on promises, these gifts should be violently, and painfully, removed. Furthermore, if their failure is spectacular, there should be further consequnses that may require players to retire a character for reasons of "Ballistica of Torment".