The Shadow Knight is the tank of the team, as you can not only strike from the shadows but soak up damage from the shadows also.
A tank that you can summon almost from anywhere that hopefully will get bounced back to hand before it dies.
For just 1 cost it has 5 health. The big drawback is that not only does it wound the card it gets summoned from, but also damages your summoner if killed. However Shadow Elves deck is prepared to nullify those drawbacks. Damaging your own units can be a plus because of Selundar’s ability and also because wounding a unit you are going to return to hand anyway is a non-issue.
Damaging your summoner is problematic, but you can return a Shadow Knight to hand if heavily wounded.
Now it may seem that defies the purpose of a tank but the reality is, if you pay 1 magic and Return to Shadow (thus returning the “card cost” and denying the bounty), you get say 3 or 4 life for 1 cost. That in terms of Summoner Wars economy, is a great return. True, you have to pay the Return to Shadow cost, but a Shadow Knight if summoned within range of Selundar provides half the cost already. The Knight can also attack and block as any unit, so ultimately if the Knight gets 3 or 4 wounds and then it’s returned to your hand, it is a great deal, making it a great tank.