Our army's stealth option, at one point they where the best infiltrators in the game nowadays they are a bit less stealthy but still good for that, lets take a look.
To start off I would say that these guys are about average for one of our melee units, which is by no means shabby but that should be taken into account when you pick their target. 3 attacks with their glimmer steel blade is solid enough but mandrake do not play as a dedicated assault unit, we have better options there.
The mandrakes ranged weapon has solid stats, and an added ability to pop mortal wounds on the target if you are lucky. It is nothing massively special, it will inflict some damage on most medium infantry, but don't expect to remove allot of MEQ with it. All that combined though does mean that mandrakes get to throw out 5 attacks over 2 phases, which isn't too bad.
Resilience wise they have a baked in -1 to hit which is nice but they almost as squishy as wyches if you hit them, flamers will give these guys a really bad time and at 15 points a model you will feel their loss.
As for options they don't have any, so that's this section done.
So their uses on the field? Well your first option is to deploy them 9 inches away from a target then run screaming at it hoping to sow chaos in the enemy line and club as many things to death as possible, multiple small units doing this could probably do massive damage to things like tau, or guard if you got first turn. You can use their fade away ability to redeploy them quickly, this can allow you to over commit, crush one flank then mop up the other faster which sounds a bit to much like grand strategy for my taste.
Small squads seem to be the way to go, MSU is unappetizing as a target and considering that you just aren't that tough, it will make them look less juicy.
All in all they feel like a unit that is a bit lacking, like it can pop up on objectives nice and easy but everything else it does can be done by someone else and done better.
Next time Incubi! <3