A flameskull was created from the head of a humanoid (usually human) spellcaster shortly after he or she had died. It needed to be fresh. They were made by dark-hearted spellcasters, typically a cleric or wizard using a create undead spell, but the precise rituals for creating them were very old and even long-forgotten. They could choose whether or not the resulting flameskull could cast additional spells. Flameskulls obviously could not reproduce or replicate themselves. Once the ritual was done and the transformation complete, the skull burst into green flames.
A group of nine early priests of Mystra once transformed themselves into flameskulls when they sought a form of immortality of their own devising, and were only partially successful with this lich-like state. They became the Circle of Skulls of Waterdeep.
Flameskulls were disembodied floating skulls, jawbone included, wreathed in an eerie blazing green flame. Their light could illuminate an area as brightly as a torch, though a flameskull could choose to suppress its fiery aura to a faint, pale-green flicker. On some specimens, the bone was inscribed with strange and disturbing runes. In their eye sockets flickered gleaming emerald lights filled with malice. As they flew, small jets of flame trailed after them. They moved in total silence, and were heard only when they spoke, cast spells, or screamed for intimidation or just for effect. They sometimes made insane, echoing laughter, but spoke in hollow, echoing voices.
If allowed by its creator, a flameskull could cast spells from their mouths. Initially, flameskulls were restricted to spells with only verbal components and they could not cast mind-control spells. Most knew up to three spells, usually magic missile, flame strike, and a long-since-forgotten incantation known as spell reflection. After the Year of Wild Magic, 1372 DR, flameskulls were reported to have a greater arsenal of spells at their disposal: detect magic, mage hand, and ray of frost at will; blur, magic missile, and ray of exhaustion each thrice a day; and even fireball and mirror image each once a day. Flameskulls after the Spellplague by 1479 DR were capable only of mage hand at will and a fireball, with great flameskulls producing a much greater firestorm instead.  After the Second Sundering of the 1480s DR, a flameskull retained full command of the spells it knew when it was alive, even ignoring the material and somatic components that it was no longer capable of but which were needed for its spells. Operating as wizards, they typically knew mage hand at will; blur, flaming sphere, magic missile, and shield according to preparation; and fireball once a day.
Initially, a flameskull could also simply vomit fire from their mouths or eyes in two gouts of up to 10 feet (3 meters) or 20 feet (6.1 meters) in a straight line, together or in quick succession. Post-Spellplague flameskulls launched a single flame ray to a distance of 50 feet (15 meters). If forced, their bite could burn their victims. Post-Sundering flameskulls shot two focused fire rays from their eye sockets, up to 30 feet (9.1 meters).
Flameskulls had all the same immunities as other undead, and were also unaffected by cold-, fire-, heat-, and electrical-based magic and attacks. They had high resistance to spells, protecting them against what they could not reflect. They were also harder than expected for a cleric to turn.
Though they could be struck by any kind of weapon, only blunt weapons damaged them effectively. Furthermore, flameskulls steadily regenerated all damage taken and could even reassemble themselves in one hour to a few days after being completely shattered.
When initiating combat against intruders, flameskulls would speak aloud and lure them into traps, or else lie to them about other dangers in the area. Those with mage hand used it open and close doors, trigger and reset traps, and manipulate other objects as needed to suit their tactics. They took pains to avoid melee and missile attacks, and focused on flying to a high place and safely hurling fire and spells down at trespassers. Nevertheless, confident in their ability to reform, they had no fear of destruction and would attack fearlessly if they must.
Flameskulls simply guarded whatever their creator compelled them to guard, such as concealed treasure hoards and secret rooms and even certain people. They were given their instructions at creation and they carried them out to the letter, so their instructions needed to be carefully worded to make sure they were done properly. However, for these undying creatures, their watches could go for years or centuries after their living creators passed away. They were usually only found in ancient places left long-untouched by the outside world.
Flameskulls retained vague memories of their old lives and could recall important periods in detail, and they spoke in their former voices. However, they were but echoes of who they were in life.
Keeping their intelligence but left quite, quite bored and very lonely by their long watches, flameskulls typically went insane and could display strange behaviors. At all times, they wanted to be entertained by whatever was going on around them and sought for some form of companionship.
They had no purpose in death but to guard that which they were created to protect. If it should be destroyed or stolen despite their efforts, or they could otherwise no longer serve their purpose, they were released from their guardianship and became independent. However, rather than attack the one responsible or seek their own destruction and escape from undeath, they would exhibit a form of gratitude and thereafter accompany the one responsible, floating just out of reach and making smart comments. And, to see if there was anything interesting happening, they would follow them everywhere—no quiet study, secret meeting, private matter, or romantic tryst was safe from the new, bright burning flameskull friends.
Although quite rare in the world, flameskulls that were encountered were solitary guardians, but they could on occasion be found in groups of up to six and even companies of up to twelve. Otherwise, they were found in the company of those capable of creating or commanding such undead.
Flameskulls could speak Common and any languages they knew in life, such as Draconic and Infernal and one or two dead languages such as Loross and Thorass. One star elf flameskull only spoke Elven. They could know up to fifteen languages, but typically only knew two.
Whenever they got the opportunity, wizards, priests, and alchemists studied flameskulls to determine how to create them or copy their powers or to identify the unique properties of their flames, though with no known successes. Due to their great age, flameskulls were apt to possess old and little-known lore and historical secrets.
To prevent a destroyed flameskull from reforming, their broken fragments must be splashed with holy water (a majority of the bone fragments needed to be doused) or be targeted with a dispel evil, dispel magic, exorcise, or remove curse spell.
In Waterdeep's Pharra's Alley, the Circle of Skulls were a regular appearance, flying in a circle and arguing with each other. Unpredictable, they aided some by giving information, but attacked those who irritated them.
- The Skulls of Skullport, unique entities sometimes counted as advanced flameskulls, as were their spellshade apprentices. The Skulls could aslo absorb mages into their mantle and transform them into true flameskulls that were vassals to them.
- Brainburn, an independent flameskull leading baneguards and dreads near the Entry Well in the Dungeon Level of Undermountain in 1375.
- A flameskull—once a star elf apprentice of Mourel Duskwalker—guarded Mourel's Tomb in the Night Realm.
- ↑ 1.00 1.01 1.02 1.03 1.04 1.05 1.06 1.07 1.08 1.09 1.10 1.11 1.12 1.13 1.14 1.15 1.16 1.17 1.18 1.19 1.20 1.21 1.22 1.23 1.24 1.25 1.26 1.27 1.28 1.29 1.30 David Wise ed. (December 1994). Monstrous Compendium Annual Volume One. (TSR, Inc). ISBN 156076838X.
- ↑ 2.00 2.01 2.02 2.03 2.04 2.05 2.06 2.07 2.08 2.09 2.10 2.11 2.12 2.13 2.14 2.15 2.16 2.17 2.18 2.19 2.20 2.21 Wizards RPG Team (2014). Monster Manual 5th edition. (Wizards of the Coast), p. 134. ISBN 978-0786965614.
- ↑ 3.00 3.01 3.02 3.03 3.04 3.05 3.06 3.07 3.08 3.09 3.10 3.11 3.12 3.13 3.14 3.15 3.16 3.17 3.18 3.19 3.20 Mike Mearls, Stephen Schubert, James Wyatt (June 2008). Monster Manual 4th edition. (Wizards of the Coast), p. 109. ISBN 978-0-7869-4852-9.
- ↑ 4.00 4.01 4.02 4.03 4.04 4.05 4.06 4.07 4.08 4.09 4.10 4.11 4.12 4.13 4.14 4.15 4.16 4.17 4.18 4.19 4.20 4.21 4.22 4.23 4.24 4.25 4.26 4.27 4.28 4.29 4.30 4.31 Richard Baker, Ed Bonny, Travis Stout (February 2005). Lost Empires of Faerûn. (Wizards of the Coast), p. 171. ISBN 0-7869-3654-1.
- ↑ 5.0 5.1 5.2 5.3 5.4 5.5 Eric L. Boyd (June 2005). City of Splendors: Waterdeep. (Wizards of the Coast), pp. 110–111. ISBN 0-7869-3693-2.
- ↑ 6.0 6.1 Greg A. Vaughan (September 2006). The Twilight Tomb. (Wizards of the Coast), p. 22. ISBN 0-7869-3947-8.
- ↑ Richard Baker, Ed Bonny, Travis Stout (February 2005). Lost Empires of Faerûn. (Wizards of the Coast), p. 119. ISBN 0-7869-3654-1.
- ↑ Eric L. Boyd, Ed Greenwood, Christopher Lindsay, Sean K. Reynolds (June 2007). Expedition to Undermountain. (Wizards of the Coast), pp. 188, 190, 191, 208–209. ISBN 978-0-7869-4157-5.
- ↑ Eric L. Boyd, Ed Greenwood, Christopher Lindsay, Sean K. Reynolds (June 2007). Expedition to Undermountain. (Wizards of the Coast), p. 13. ISBN 978-0-7869-4157-5.
- ↑ Eric L. Boyd, Ed Greenwood, Christopher Lindsay, Sean K. Reynolds (June 2007). Expedition to Undermountain. (Wizards of the Coast), pp. 48, 69, 80–81. ISBN 978-0-7869-4157-5.