As you're probably aware, you're not the only hunter/assassin in Warframe. There are others hunting you.

An hour, a day or a month after you get an initial message that you've been marked for death, you'll be visited by an uninvited guest. The warnings are terrifying, the lights flicker, there are threatening messages... and then you get the Assassin:

Stalker (first name "What")
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"I hunt, dividing your numbers. Watching from that dark place, cataloging your sins
I am the ghost of retribution. You may forget but you are not innocent."

Referencing itself as a "Low Guardian", and despising the Tenno with its very being, it appears as a shadow assassin, with deep black and red colors, veiled by black smoke and some pretty terrifying gear. It wields a bow and arrow (Dread), a two-handed scythe (Fear) and throwing knives (Hate) and has a wide range of powers. It only goes for a single target, not caring of other squad-mates, and if it succeeds in bringing him/her down, it will disappear in a stance of meditation. Nobody knows too much of the Stalker, but it is more than a myth. It is your doom.
Defeating it will give you blueprints to all of its special weapons, as well as a rare or uncommon mod.

The Grustrag Three (Insane Super-Soldiers)
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"You are going to suffer for that, Tenno!"

Composed of Vem Tabook (leader), Shik Tal (shield) and Leekter (hammer), these guys are the most absolute dreadful threat if you're playing alone. The only hit-squad that features more than one killer, they're all large, powerful and fierce, and they have two sentinels to back them up. That is why, unlike in other intrusions, when they are breaching one of your missions, Lotus asks you to abandon your current objective and run, as fast as possible, directly to extraction. Then the comms break.
Another trick to them is that once they cut you down, while you're in bleeding mode, they trap you with an energy cage, blocking you from further firing your guns, dying (so you can't use a revive) or being revived by team-mates. The force-field can only be disabled if the 3 Grustrag are all dead.
Also, as an added element, once you're taken down by them, you get tagged with a Bolt, a device that limits a Tenno's effectiveness against the Grineer. It visibly shows attached on the right calf of your Tenno until you craft a releasing device to remove it.
Upon their defeat, they drop a Brakk component and a rare mod.

Zanuka Hunter (aka The Harvester)
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"Thank you for volunteering for the Zanuka Project, you will make a fine addition to my collection."

A quadropodic Robot beast with high mobility and a wide range of weaponry. It resembles the Hyenas, but is smaller, quicker and harder to shoot, and it descends from the Zanuka Project that Alad V started to harvest the Tenno powers against them. It is dispatched if you support the Grineer against the Corpus on too many missions, and it will follow only death-marked player. The meeting ends when either the player or the Zanuka dies. It has a chance to drop Detron Blueprints (a one-handed fire-arm) if you slay it.
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Who is your favorite one? Who did you escape most often?
What other Assassins would you add if you could make one up?
What would be their powers and affiliation?