This material makes a character capable of passive invisibility, so DMs should take note of that fact. Situations which assume that the character must make effort to obscure their presence to remain unnoticed may not be significant challenges to a character using this material. Challenges of these sorts are, however, an excellent tool to highlight the character's advantage. One should be cautious to include challenges which are actually capable of threatening such a character regularly; such as creatures that can sense the character's smell, heart beat, or thoughts; or environmental effects which would reveal them like ambient smoke/steam/dust, being covered in ink, tripwires and alarms, or barriers which must be moved- like doors; but without punishing the player or coming across as a gimmick. Be careful about who is allowed to use this material, as immature players will likely abuse this to derail the campaign. As should be standard, such players should be dealt with outside-of-game through open communication, but it is often best to avoid the situation in the first place. New players often have difficulties visualizing themselves as another person, let alone another creature capable of existing in a state that they themselves will never experience. As such, if a new player is allowed to use this material, they may need some degree of guidance to remember that their character can go unseen without effort, and may in fact be invisible at any given moment, though it should only take a few sessions to really drive the point home.