The Shadow Assassin's are equivalent to the Gray Men or Soulless

Potential Spoilers Below

Gray Men, also known as Soulless, are creatures of the Shadow used as assassins. Unlike most types of Shadowspawn, however, a Gray Man is not a construct. They begin as ordinary men (and women, though allegedly rarely) who have not only pledged their soul as a Darkfriend but given their souls to the Dark One. After this process, they become almost completely unnoticeable. While the Gray Men are not invisible in the literal sense, even the gaze of alert eyes will slide right over them.

Gray Men, said to be ordinary-looking to the point of being literally beneath notice

A Gray Man slipping by an Aiel guard, unnoticed

The Dark One attempting to break into the world of man

They are highly effective assassins because of their ability to pass completely unnoticed, whether in busy public areas or infiltrating heavily guarded private quarters. The victim himself generally does not realize the presence of a Gray Man until he is struck. Witnesses to the murder often report seeing the victim fall and yet still failing to see the Gray Man killer.

In the A Song of Ice and Fire novels, Melisandre actually conjures forth two shadow assassins on separate occasions to kill Stannis' foes - Renly and Ser Cortnay Penrose. She needs to use Stannis' own life-essence to fuel the entities, but this hugely drains him, thus she doesn't produce any more shadows. Davos notices that Stannis looks ten years older than before the Battle of the Blackwater. Use of the creatures is thus limited in case their creation taxes the creator to the point of death. Melisandre offers Davos to use his life-essence to produce a shadow, but he refuses. It is unknown whether Melisandre made similar offers to any other men.

Melisandre birthing a shadow assassin

Melisandre birthing a shadow assassin

Melisandre birthing a shadow assassin

Shadow assassin killing Renly Baratheon 

Stannis Baratheon

Davos Seaworth

I simply see that the word soulless and life-essence as being the same thing.  Neither really has a soul and both are created from ordinary men.  Did the creation of the assassin in the Game of Thrones actually take a little bit of the soul as is done in the creation of a Gray Man who gives up his soul to become one of the Dark One's assassins?  Davos' comment about Stannis looking ten years older seems to suggest this to be the case.  Both the Gray Man and the Shadow Assassin are barely noticeable to the one's who are their intended targets until it is too late.

Comments encouraged.  Love to hear the idea’s of others.  Most believe that since I present my idea’s as “fact like” I’m not open to change my viewpoints which is far from the truth.  I simply look at the information presented and go from there.  If you can shine a light on another way of thinking that opens the door to debate.