Harvester (Skyline)

The skyline aliens are black cephalopod-like creatures with the ability to transfix and utilise the brains of different species to power themselves.

Varieties
There are four known types:

Drones are squid-octopus like aliens with multiple eyes and glowing lights on their tentacles that they use to bring other species into a trance-like state. Drones have the ability to steal the brain and spinal cords of their prey. Drones have slimy black skin.

Tankers are giant crab-frog like creatures with multiple eyes and 8 brains for "power". Instead of hypnotising prey with blue lights, they have clingy tentacles which they use to hunt. They walk/run on four legs and are olive green in color. They are incredibly strong but their skin is flammable.

Aerial drones are black flying aliens that function as dropships. They patrol regions being attacked and release drones to find prey. They have wings and one big blue eye, but lack the power to 'hypnotise' their prey. They are used like Kamikazis or "suicide bombers" when combating enemy aircraft. However, they are decimated, it is even safe to say that they are massacred, by the US Air Force's Reaper drones during a dogfight, in which the alien mothership is destroyed by a successful nuclear attack. 

Worker/guard/soldier? are quadrupeds which resemble smaller tankers. They have melon-shaped heads with murones. Every individualltiple blue eyes and slimy, black skin like the drones needs a stolen brain in order to function.

Abilities and anatomy
Most of these aliens have the ability to hypnotise a host and feed/power up by "brains". They have a blackish-purple blood (the purple hints at haemoglobin). A bright blue light they generate seems to entrance creatures, and the aliens suck prey into their body through holes or mouth-like orifices. Only the Drones have the 'power' to transfix in this way. This is unlikely to be an evolutionary trait, due to its sophistication and seemingly impossible effect. It could only be technology that they developed. They are able to regenerate lost tentacles with great speed.

Technology
Their technology is a fusion of metals with some organic components, and some of their machines seem to be able to rebuild themselves even after a nuclear strike, though these might be destroyed if the explosion is in the interior.

This is a list of machines:

Their ships are made with a metal hull and organic substances on the inside, a tractor beam, and the device that enables the aliens to hypnotise. They look saucer-like, but with spirals and arches sticking out.

Their abduction devices are tractor beams, accompanied by a bright blue light, that can suck Humans into the air, and possibly descend from their ships.

The 'alien sorter' is a thin robot with a blue light on the front and a 3-pronged claw, razor blade, and a tube that sucks up the brains into a room that attaches it to the aliens.

Although they do not have any type of small arms or ship firing guns (with the exception of the Aerial Drone as it is a "living missile") they are advanced technology wise.