malefic grasp can go and stay go.
nightfall should be baseline and stack charges.
they need to bake the drain soul damage and sub 20% damage bonus into drain life, along with the soul shard refill upon death-while-afflicted.
i'd even give a castable-while channeling drain life "shadowburn" called soul tear that does a small portion of damage but mostly adds healing absorb spending as many soul shards as the warlock has for more damage.