TEXT: Shadow Walker Hosts are utilised when the Eldar need to
  insert well-armed scouts deep into enemy territory. The War
  Walkers of the host range far and wide, utilising a unique
  cloaking field system called the Cloak of Shadow to remain
  hidden from enemy sensors for days, even weeks, at a time.
  When the main force of the Eldar army is ready to engage
  the enemy en masse, the Shadow Walker Host takes
  position, often ahead of the army or on the enemy’s 
  flank. At the key moment, as directed by the Craftworld’s
  Farseers, the host will disengage its Cloak of Shadow and
  concentrate all its fire on the enemy, trapping him in a
  lethal crossfire pattern referred to as the Web of Twilight.
  Few enemies survive long enough to report the presence of
  the Shadow Walker host, most being cut down within
  seconds by the intricate pattern of death.
  SHADOW WALKER WAR WALKER HOST
  POINTS: 50 + MODELS
  FORMATION:
  1 War Walker marked as the command vehicle.
  2+ War Walker squadrons. 
  SPECIAL RULES:
  Cloak of Shadow: The formation is not deployed at the start of the
  battle and is not placed in strategic reserve. At the beginning of any
  Eldar Movement phase, nominate a point on the table more than 12”
  from an enemy model, and place the command vehicle on this point.
  The remaining War Walkers are then placed within 6” of the command
  vehicle.
  Web of Twilight: In the shooting phase of the turn in which the
  formation is deployed by way of the Cloak of Shadow special rule,
  each War Walker may fire each of its weapons twice.
  “As a silvered moon is concealed by cloud, so we bide
  our time. And when our face is revealed, our enemies
  turn and flee before the harbingers of their doom.”
  Sarithel Jhelisdai of the 
  Moonstrike Shadow Walkers
  COMMAND WAR WALKER
  WAR WALKER SQUADRON
  WAR WALKER SQUADRON
  Dahaidith (Silent Voice)
  Tahliadeth (Eye of Shadow)
  Y'lintil (Beholder)
  This Shadow Walker host fought throughout the war
  on the Moon of Delith (called Rancour 12 by the
  Imperium). Its most audacious and decisive action
  was against the Adeptus Mechanicus research site at
  the moon’s south pole, where it infiltrated past the
  humans’ automated defence points to engage and
  destroy the enemy’s command centre.
  
SOURCE: textdata/thevault/Warhammer/40k/1st-8th Editions (Prior)/5th Edition (2008)/Apocalypse Datasheets/40k 5e - Eldar - Shadow Walker Formation.pdf