I - Rules
(see TOI, p. 7)
When a figure with the Stealth ability is attacked, the attacking figure must roll the transparent stealth die in addition to the dice that figure rolls normally.
If the result of any die used in the attack roll is an "X", the attack is a miss.
When a single attack roll would affect multiple figures (for example, an attack using Blast, Breath, or Sweep), and any of those figures have Stealth, a single stealth die is included in the attack roll, but the stealth die's result is used only for the figures that currently have Stealth.
Multiple sources of the Stealth ability do not stack. A maximum of only one stealth die is rolled for any given attack.
Example: Okaluk and Rakash start the game with the Stealth ability. Even if Okaluk and Rakash drink an invisibility potion, any figure attacking Okaluk and Rakash still only rolls one stealth die in the attack roll.
II - References
Heroes with the Stealth ability: Okaluk and Rakash.