Triarch Praetorian
Triarch Praetorians are the equivalent of Assault Marines, being Necrons with anti-grav jump packs. They're also pretty valuable as agents of the ruling body of all Necrons, the Triarch.
Why it Sucks to Be Them
Following the brouhaha that is the War in Heaven, the Necrons decided to go to sleep for fuck-all many years. During that time these chumps, becoming the emo that is 5E Necrons, the Triarch Praetorians decided "Fuck sleep! Let's go guard shit as penance!" and kept a VERY long watch on the hibernating 'crons and the protection of their culture. Apparently they were good enough that even spess elves couldn't screw them up despite literally millions of years spent trying.
During this time they taught some of the younger races the ways of the Necrons, as they figured keeping their culture alive is good enough to fulfill the Silent King's final order.
However, now with their brethren awakening they decided to slam the alarm on as many Tomb Worlds as they can, making sure every lazy bum gets their beauty rest ruined. Being the special agents of the ruling council, they uphold the rule of the Triarch and see to it that the traditions of the Necrons are followed, disposing of any who challenge them with a single blast of a Rod of the Covenant. This means they are effectively Alien Zombie Robot Commissars. I believe I do not need to explain why or how this is awesome.
Their Usefulness
For those familiar with the workings of the Shining Spears, the Triarch Praetorians will be familiar. They are expensive, fast, tough(ish) and hit hard yet they lack the Attacks to make themselves count. Shooting and striking at Str5 AP2 they are likely to plow through many individual targets... but since they only swing twice on the charge AND they strike last (Unwieldy is a bitch) they are not likely to kill their targets outright. In return they are not particularly tough and can be swarmed or just beaten to death.
Welcome to 7th edition. Praetorians are cheaper, go at Initiative (yeah it's only 2, but that means you go before Power Fists and Thunder Hammers), and have 2 attacks base, 3 on the charge. With the new Reanimation Protocols, they're tougher than ever and can even laugh at things with AP3 attacks. Assault Marines and Vanguard Vets wish they were this awesome.
In their alternate loadouts they gain 1 Attack, Rending and Entropic Strike. Paired with their high mobility this means that they can make effective (but expensive) tank and skimmer hunters. Scarabs tend to do better on this part, but if you want to use the Triarch, go ahead, as they're much tougher in the new edition.