Stalkers become available when your commander has completed the stalker research which becomes available when the lair reaches level 2. They are significantly stronger and more powerful than the Scavenger. If you plan on taking down buildings early, a Stalker is a great choice with a 50% damage modifier over and above their base damage rate. For their cost/damage ratio, they also have the advantage in straight melee damage over other non-siege beast units having a 20% higher unit pierce modifier over the more powerful Predator. They do the most melee damage of all the main beast units. Simular to the Savage, who does the most melee damage of all the main human units.