Bastellus remained unseen until it was time to feed. People who survived the encounter or witnessed its feeding described bastellus as a hulking humanoid made out of shadow. Their shape lacked any distinct features. They could change their shapes to blend better with surrounding shadows or intimidate. The most sinister thing about a bastellus' appearance is its leering smile. The creature's long twig-like fingers sprout tentacles that wrapped around the face of their victims during feeding, truly a horrifying sight.
The incorporeal nature of dream stalkers made them mostly invulnerable to physical attacks, only being affected by weapons with a +3 equivalent or more powerful enchantment. However, bastelli were still resistant to these powerful enchanted weapons. Cold, fire, or lightning damage did not affect these creatures as well as charm, sleep, or hold monster spells. Being incorporeal, dream stalkers were immune to all poisons.
Bastelli could be affected by turn undead, but holy water posed them no threat. The only way to destroy one of these creatures was via the dispel evil spell, but even then, a bastellus could avoid complete annihilation.
When hiding or moving through shadows or dimly lit areas, bastelli were virtually undetectable. Dream stalkers were repelled by the protection from evil, did not enter areas affected by it nor attacked creatures under the effect of the spell. The negative plane protection spell was effective against bastelli and could scare it from feeding on a creature for good unless compelled to.
When feeding, dream stalkers placed their hands on the sleeping creatures' forehead. During feeding, bastellus experienced extreme pleasure as they threw their heads back in ecstasy.
Bastelli, being creatures of darkness, avoided lit areas. The light did not hurt them but could help to keep the undead at bay. Bastelli used their ability to create darkness to extinguish magical and mundane sources of light in order to reach their sleeping victim. However, their darkness could not dim the light from more powerful sources or created by more powerful spellcasters than bastelli themselves.
Bastellus' main tool was its extremely powerful version of sleep spell that affected all waking creatures within 50 feet (15 meters). This sleep affected elves and half-elves who were normally immune to it. 
When bastellus needed to attack, it did so throughout the sleeping person's dreams. Upon the creature's touch, the victim's dream became twisted with evil and horror. The nightmares inflicted by bastelli were pulled from the dreamer's own subconscious mind, showing the greatest and deepest fears. The prey felt shaken and distressed upon waking, remembering the psychological torment of the dream. They did not receive benefits of resting, such as healing or spell memorization, and were too distraught for the following day to keep the concentration required for spellcasting. The most terrifying effect of bastellus' feeding, however, was the power and experience drain. The victim woke up significantly weaker than they were before the feeding.
Most of the time, a bastellus preferred to flee from battle or use its stealth, as their sleep spell could only affect creatures who can not see the dream stalker. Their ability to create a 50 feet (15 meters) area of darkness along with the ability to pass through solid matter were bastellus' main ways of escaping.
When forced into open confrontation, bastelli attacked by moving through their opponents, causing them to fall into a state of extreme paranoia. This was the effect similar to how bastellus affected the prey's dream. The creatures' touch pulled insecurities and fears from the target's subconscious mind, making them see their companions and friends as enemies. That confusion usually lasted from six to twenty-four seconds, but the chaos created was enough for the creature to escape.
Bastelli were rarely-seen solitary creatures. They could mostly be seen around sleeping victims. They were drawn to places with high populations, lush with brewing prey.
Successful communication with bastellus was extremely rare, making many believe that these creatures did not have a language that was known to mortals. However, they could communicate in common when summoned to serve a master. Some believed that dream stalkers could communicate via messages when manipulating dreams.
Bastelli were attracted to the individuals with the most vivid dreams of creative individuals or passionate and motivated creatures. Once an individual dream stalker fed on a person's dreams, it became obese with the ecstatic taste and returned for additional feedings until the target's death.
Using their sleep spell to hunt, bastellus moved onto its chosen victim once all creatures in the vicinity were incapacitated. The bastellus' sleep spell plunged its targets into a dreamless slumber, forcing the undead to only be able to feed on creatures who fell asleep by other means or naturally. For some mysterious reason, bastellus was not able to feed on elves and half-elves.
A creature drained completely out of their essence in their sleep appeared to have died of a heart attack. If the victim's body was not destroyed, its spirit came back to unlife several days later as a newborn bastellus. Each newly created bastellus shared no bonds nor connections to the dream stalker who spawned them. Newborns had no memory of their previous lives and were guided by their burning hunger for dreams, the only way they knew how to fill the emptiness inside them. If a newborn bastellus failed to feed during its first night of unlife, it faded away and ceased to exist.
When bastelli were stricken down, they dissipated into the air like steam escaping the kettle. However, if they were not destroyed via dispel evil, they eventually reformed to feed on the sleeping. When dying bastelli released one final telepathic assault filled with pain and sorrow that affected everyone within 50 feet (15 meters) similarly to a fear spell. If a dream stalker was defeated during its feeding, the prey was affected by the creature's death, and its system shock could easily kill the dreamer. When defeated but not destroyed, bastellus reformed in the place of its birth one day after. On the Demiplane of Shadows or the Demiplane of Nightmares, bastelli could swiftly recover any injuries they suffered before returning to the Prime Material plane.
Sages theorized that bastelli embodied aspects of both Negative Energy plane and the Domain of Dread known as Ravenloft. Some sages postulated that bastelli were a byproduct of the living world, personifying creatures' unfulfilled hopes and aspirations.
The dream essence of bastelli was priceless to necromancers for its potential in creating powerful magical items, and illusionists. When used as a material component for illusion spells, dream essence empowered the effect, making it exceedingly life-like.
- Sigh: a bastellus from another world, summoned via curated magic of the pool of twilight that served Sirana and her in 1372 DR on her quest to find the Hammer of Tyr.
- Pool of Twilight
- William W. Connors (1996). Monstrous Compendium - Ravenloft Appendices I & II. (TSR, Inc.), pp. 10–11. ISBN 0786903929.
- James Ward, Anne K. Brown (November 1993). Pool of Twilight. (TSR, Inc), chap. 4. ISBN 0-8803-8735-1.
- Beth Bostic (2002). Ravenloft: Denizens of Darkness. (Wizards of the Coast, Arthaus, Sword & Sorcery Studios), pp. 22–23. ISBN 1588460770.