As a master demo lock, you can sac your succi and get +15% shadow damage. Very handy. There has been a long time bug, where you could also get your pet combatrezzed, thus gaining the 15% for the sac and the 10% for having the succi alive. The trick is to have either a druid, or someone with jumper cables rezz the corpse before it rots OR have a resser spam their 10 second rezz as soon as you sac the pet (the pet rots after 10 secs, the rezz has a 10 sec cast time).
Now, there is also a twist to this technique: Shield (priest) your succi and sac it. The succi is unaffected, and you gain the +15% from the sac and the +10% from having her alive. This will be fixed in 1.12, but I highly doubt the first technique gets fixed.
Enjoy 