There is a critter flag “OCF_IS_MOVING_SILENTLY” which indicates Sneaking state of any NPC/PC. Pressing Sneak button in ToEE Menu would execute standard built-in action – “D20A_SNEAK”, which would call “PerformSneak” function in T+. PerformSneak The function is intended to check, whether NPC is allowed to stay in the Sneaking state. If current mode is no combat then yes, no check. When Combat, then -20 penalty is added. Skill Hide is rolled for this NPC with the penalty applied. Then for each combatant which is foe, who has Line of Sight of NPC check is done – if NPC does not have “Hide in Plain Sight” feature then fail, otherwise Spot roll for foe compared to initial Hide roll of NPC. If at least one foe spotted NPC, Sneak state would be removed, and failed roll would be printed in Combat History window. Success rolls, “Hide in Plain Sight” inquiry, etc does not print in Combat History window. Overall even “Hide in Plain Sight” feature of the NPC is insufficient, as -20 penalty is extremely high. Sneak_Attack condition A Rogue class applies "Sneak_Attack" condition on NPC instance, which post-processes damage of successful NPC attack, and would grant additional damage when NPC is in Sneak state or other circumstances. Essentially any time Rogue deals damage, "Sneak_Attack" would check, if there are circumstances, that allow adding Sneak Damage, and they are the following: Target is Flanked NPC is Sneaking and Target cannot overcome it Target cannot sense NPC (LOS, Invisibility) Attack is Critical, and NPC has “Telling Blow” feature If any of these circumstances pass the Sneak Damage is calculated and applied. Sneak Attack Bonus There is no implementation of Sneak Attack bonus in ToEE. Sneak Action Cost In ToEE and T+ Sneak is Move Action cost.