Rank 1 - You no longer set off enemy mines. Ranged sneak attack do 2.5x normal damage and your melee sneak attacks do 4x normal damage.
Rank 2 - Running while sneaking doesn't affect stealth. Ranged sneak attacks do 3x normal damage and your melee sneak attacks do 5x normal damage.
Rank 3 - You gain a Chameleon-like ability when completely still and sneaking. Ranged sneak attacks do 3.5x normal damage and your melee sneak attacks do 8x normal damage.
Rank 4 - Engaging stealth causes distant enemies to lose you. Ranged sneak attacks do 4x normal damage and your melee sneak attacks do 10x normal damage.
Rank Challenges
Rank 1 Challenge Progress - Spend a total of 12 Skill Points in the Physical tree.
Few skills capture the imagination like the ability to remain undetected, used by assassins, special forces operatives, and simple thieves alike for centuries.
Concealment
Concealment is a Master Physical skill.
It can be unlocked by spending a total of 12 skill points in the Physical skill tree.
The fastest way to quickly progress through the skill rank challenges would be to use EM weapons to knock out creatures, then stealth attack them with a melee weapon.