Ymgarl Genestealer appear from nowhere

The moons of Ymgarl are the supposed origin of the Genestealers. Though the Salamnders and the Inquisition pressed hard to purge them, Genestealers are not only infiltrating space ships and space hulks but are also nowadays commonly used as infiltrating and outflanking troops by the Hive Mind.
Hive Fleet Openvalley has recently produced six of this special brood of Ymgarl Genestealers.

In the game they appear in a piece of area terrain nominated before the game. In each assault phase they can alter their form to receive +1 to Strength, Attacks or Toughness.
Let's how they perform on the battlefield!