Scarabs (Scarabaeus sacer) are a species of Dung Beetles native to the Mediterranean region. Said beetles are notable for making spherical masses of dung which they lay their eggs in and roll into protected burrows as well as having significance in ancient Egyptian mythology.
Necron Scarabs AKA Canoptek Scarabs are beetle like robots about 30cm (roughtly 11 inches) long capable of limited flight and able to disassemble and reassemble matter at a molecular level. Scarabs have a variety of functions, ranging from maintenance of Necron bases and machinery to combat. While an individual scarab can be put down with a good lasgun shot, they generally travel in groups. A swarm of Scarabs can eat a Leman Russ Battle Tank and its crew and crap out a pile of slightly damp metallic dust with some carbon and calcium impurities in seconds. Give them some more time and they can also crap out the tank's mass in new Scarabs. Scarabs are not sapient, they are generally slaved to either Necron overseers or Spyders. If neither is present, Scarabs tend to become mindless von Neumann Machines eating everything and making more Scarabs until you have a planet with its entire surface covered with the little bastards. Given enough time you won't even have a planet, just a planet sized ball of scarabs.
Crunch-wise, their statline is pretty worthless but they're very fast and very cheap, making them useful for tying up shooty units for a couple of turns or interrupting an objective rush.
Flensing Scarabs are a special type of Scarab meant to strip flesh from bone. They are much smaller than the common garden variety, being the size of a beetle. However their small size doesn't stop them from OMNOMNOMing your largest Tyranid or Squiggoth in mere minutes.
Mindshackle Scarabs are a even more specialized type of Scarab. As their name implies, they are very small Scarabs that burrow into the victim's skull for ASSUMING DIRECT CONTROL of the poor fool, turning them into a mindless slave.
They were squated out of 8th edition but a home rule that is true to the fluff (it's rules in 7th weren't) might read as follows: May be equipped by any NECRON character except C'Tan Shards. Anytime the equipped character scores a hit (but not necessarily a wound) on any non-vehicle unit in close combat (after resolving for wounds or damage from said hit) the attacking player may elect to roll 2D6, if the total is greater than the victim unit's highest leadership value the unit is under the control of the attacking player until the end of his turn. If the total is less than or equal to the unit's highest leadership value the unit does not switch sides and it is immune to Mindshackle Scarabs for the remainder of the game. If the unit does switch sides repeat the leadership test at the end of the controlling player's every turn that the unit is under the influence of Mindshackle Scarabs. Only one unit may be under the influence of any one character's Mindshackle Scarabs at a time. The controlling player may elect to give up the controlled unit at the beginning of his turn in order to attempt take control of another unit during close combat, a unit given up this way is not immune to Mindshackle Scarabs. Costs 20 points.
The initial tomb awakenings produced larger, more specialized Scarabs. These creatures were little more than bombs with legs that crawled over the target, seeking weak spots to exploit. Against infantry, this typically involved clamping onto the target's face before exploding their head in a shower of gore.