If you're gonna take Mandrakes, take at least 10. They're good at killing infantry in cover, good screening if you need it, good for chipping mortal wounds off something with an invuln (demon prince), all around a super good unit. They're a good counter unit as well, since they're good against things that are going to get in your face like demon princes, Blood Angels, that kind of thing. Wait for them to come at you, then drop and shoot.
Overall super versatile. Expensive though, but they fit well into a Venom spam list if you ask me.
Yo ho, yo ho, a drug-fueled BDSM space-elf pirate's life for me!
Can I get a Roll Tide?