I did several tests.
I have the impression that this node does not work.
If I understand the skill description correctly, with 3 points invested in, I should get + 3% to all buff effects provided by Dread Shade for every 3% of health lost by the minion.
Which means, for example, that the damage bonus would go from + 25% to about + 47% if my minion lost 90% of its life (of course such a bugg is supposed to be a very temporary buff, since a minion that has reached 10% health will die very quickly.).
However, I do not observe this increase in damage.
I also do not observe an increase in attack speed when my minion’s hit points are affected by a Shade.
(The Dying Coven node should give me + 45% x 1.9 or + 85%).
However, I did not observe any increase in attack speed either.
So I have the impression that this node is currently bugged, and does not work on damage buffs, nor on other more special buffs (Divine Coven, or Martyrdom for example). 