Bring the fear!

While not the most straight forward ability to use, the obsession of the Dark Creed to give a -1 to the enemies LD could be absolutely amazing against a lot of armies, especially if you take it for some of our units that you would take anyway.  
The key strategy here is getting 3 units within 6" of a unit and then also hitting them with a PGL which can give -4 LD to an enemy squad.  On top of this if 1 single wound is done to the enemy unit they are taking a test on a -5!
But what are the best units to try and get this on?  I would say, Raiders.  Raiders don't do much in a fight other than tie up an enemy force, so when wyches are doing their thing, and carving up the enemy, the raiders are pretty useless.  This gives them that little bit of a punch in an additional phase, without losing anything from a lot of the obsessions.
As a side note the Heamonculus makes all the raiders tougher.

The obvious choice is a patrol:
1 Heamonculus
1 squad a wracks
1 Raider with PGL

This of course forces us to take a squad of wracks (if you are like me you weren't going to include them).  So there is tax here, although they do make wonderful objective grabbers with their higher toughness and the access to the electro whip shouldn't go without note.

This can also be optimised by taking an extra raider if you want to spread the fear even wider, which would be:
1 Heamonculus
1 squad a wracks
2 Raiders with PGL

Scourges can be added to this detachment to gain access to additional Raiders to give us access to more of this ability.
1 Heamonculus
1 squad a wracks
2 Raiders with PGL
0-2 Scourges
0-2 Extra raiders with PGL


Another option if wracks aren't your thing, and you are going to run a scourge heavy heavy weapon battalion is:
1 Heamonculus
3 Squads of scourges
4 Raiders with PGL

Incubi and grotesques can also be added here for the vanguard detachment or Taloi/Cronos can be used for a spearhead (unfortunately ravagers and fliers can not be coven units)

Note; that venoms may also be taken if you'd prefer, but with the access to the PGL, for me this makes raiders superior, at least for the first couple of buys.

While not useful against some armies, the opportunity cost is relatively minor considering the fear that we can bring to the table!

I'd probably keep it simple and go for a Vanguard of 1-2 Haems, 3 x 4 Grots and 3 Raiders with PGL. You want your Grots up close and personal anyway so if you throw them into the middle of the enemy they should be able to reduce the Ld of several units and hopefully cause at least one casualty to each unit in your fear radius for a morale check at -5 for each unit.