If the debuff is 7.5k per DK, which with four DK's adds up to 30k.

Ideally, you'd want each DK to apply the debuff immediately.
And to use the other Unholy rune to apply the debuff again, at the 5 second point.
By the 10 second point, the first rune will have regenerated, which means another application.

So, Necrotic Strike will negate the first 30k in healing, every 5 seconds.



That means two attack sequences, really.
One where we want to put out an AoE shit-storm.
So we prioritize Howling Blast, over everything and Frost Strike as secondary because that can regenerate a depleted Frost Rune (or randomly another rune type, but if the DPS priority is Frost Runes - Runic Power - Other Runes then most likely a Frost Rune).

The second type of sequence is whenever we want to focus on something.
So 4x Necrotic Strikes every 5 seconds, as top priority.
And then whatever is the most DPS.