Not a Drk, but... Why not give them a magic specific Subtle Blow trait that by 99 stacks high enough to lower enemy TP gained from a Drk's spells to 0. Then they could sit back in big events where it's not safe for DDs to go in and cast magic without feeding TP to the mob and killing the tanks. And it'd work with their Occult Acumen traits to let them build TP for when it's safe to run in and damage.

It's situational, but one situational use is more situational uses where their magic is worthwhile than they have now.