Fancy little scenario

The Party’s Rogue discovers a hidden passageway within the Tomb of Korvath and group continues down into the dark.  After 40 feet or so, they come upon a medium sized chamber that seemed to be study of some sort.  You see the walls lined with bookshelves, housing untold knowledge with its plethora of ancient scroll and texts.  At the opposite side of you lies a large desk covered in parchment, burned candles, black ink, and a small chest placed in the middle.  The Rogue, with his eye solely on the chest, moves towards his booty, and subsequently, fails to notice three small dusty skulls laying plainly under the desk.  As he approaches, the three skulls engulf in green flame, its eye sockets glow in a deep reddish-orange glow and begin to fly about the room chittering.  In mad laughter, the skulls illuminates the secret study in Green flame while rays of fire appear from both of the empty eyes of the creature, burning the Rogue.

THE FUCK IS A FLAMESKULL?

A Flameskull is a human skull that is engulfed in green flame.  Yep, that about explains the physical aspects of these cute little buddies.

In DnD lore however, flameskulls are the long dead skulls of wizards, hence why they have cool spells to fuck up your party’s day.  When they are resurrected or transformed, the flameskull only somewhat remembers its past life, in a way that echoes the life that once inhabited it much in the way that a player casts “Speak with Dead.” They are eternally bound to serve its creator in order to protect some valuable goods, a hoard of treasure, a secret room, or even as bodyguards if you like floating flaming skulls cackling around all day.  They are intelligent to a degree but take their commands very literally, so be careful with how you use them or else you can find yourself regretting ever summoning these little fuckers to begin with. These undead chitter chatters can even rejuvenate once destroyed unless they are doused in holy water, if you cast dispel magic, or cast remove curse. 

LET’S LOOK AT THE NUMBERS

These tiny constructs are decently easy to hit with an AC of 13 but are actually pretty beefy for a tiny skull with 40 HP and can fly around at 40 ft.  What really makes these little guys tough to kill is its innate damage resistances and immunities.  They are resistant to lightning, necrotic, and piercing damage, while also being immune to cold, fire, and poison damage.  Did I forget to mention that they immune to being charmed, frightened, paralyzed, and poisoned?

STR 1 (-5): They are tiny skulls.  What do you expect?

DEX 17 (+3):  Makes sense.  They can fly around and are tiny skulls so hitting them or trying to catch it with your handy dandy skull catching net is gonna be tough.

CON 14 (+2): As an undead creature with several resistances and immunities, it makes sense that it would have a higher than normal constitution. Is magical constitution a thing?

INT 16 (+3):  It’s the skull of a long dead wizard, of course its hella smart. What, you think it’s just going around laughing and screaming like a 2-bit haunted house where you can smack it around like an undead piñata?

WIS 10 (+0): Can make reasonable decisions on its own. Little buddy ain’t stupid.

CHA 11 (+0):  Honesty think this should be somewhat higher for an undead being wreathed in mystical green flame that attempts to kill anything in sight.  That power of intimidation yo.

WHAT MAKES THEM COOL AS SHIT

Illumination: While being wreathed in awesome ass green flames, they illuminate the area around them in a 15 foot radius making them the perfect creature for your haunted mansion hidden by a deep fog at midnight in the middle of nowhere.

Magic Resistance: it has advantage on saving throws against spells and magical effects.  Combine that with its resistances/immunities and goddamn it’s a spell casters worst fucking nightmare.

Rejuvenation:  Why won’t the dead just stay dead?  Once destroyed, it regains all its hit points in one hour.  Perfect for that early dungeon encounter they forgot about, and once, after destroying the BBEG of the dungeon, they return to exit the dungeon, “FUCK WE SHOUDL’VE TAKE A SHORT REST OR SOMETHING WHY ARE THEY ALIVE?!?”

Spellcasting:  Well yah, they used to be wizards rights?  They have a good balance of both defensive an offensive spells which will further infuriate your players.  The ability to cast shield giving itself a +5 bonus to their AC as a REACTION, to cast Blur as a concentration that forces the player to hit you at a disadvantage, then can turn around an throw fucking Magic Missile at you, and then to further fuck your players up, it can casting motherfucking FIREBALL.

HERE ARE SOME AWESOME ASS IDEAS ON HOW TO USE FLAMESKULLS

Awesome ass ideas

1.  Let’s start with the obvious.  Flameskulls are created to protect and guard something precious to the creator.  So, following that simple logic, have the flameskull protect a secret room full of his secret stuff, a hoard of treasure in their cellar, or as creepy ass lanterns on Halloween.
2. Branching off that first idea; when protecting a hoard of treasure, you can easily use a flameskull as a small ally to a much larger beast, making an already tough encounter tougher.  Have a flameskull inside of a Shield Guardian’s helmet, therefore creating a cool offensive and defensive combo as “one” creature. 
3. Flameskulls are the skulls of long dead wizards right?  Let’s have a wizards college dropout dig up some wizard graves and create some flameskulls to attack the college as revenge, or defend his hideout as he tries some ritualistic magic to get back at all who laughed him!  VICTORY WILL BE HIS!  SHOW THEM WHO THE STRONGER WIZARD IS NOW ASSHOLE!
4. A governor of some sort who wants move up the political ladder kidnaps some poor princess-like girl who is just minding her own business singing to birds and whatnot.  Bad guy puts her in tower and guards her with flameskulls.  A bit classic fairy tale-ish but I think it could work. Especially since I’m coming up with all the good ideas you lazy-ass.
5. This would be a random encounter that you could use rather than as part of an adventure.  Flameskulls lose their purpose once their creator has past one from the land of the living.  Therefore, have a few raving flameskulls “terrorize” a small village by floating around listlessly in the nearby woods at night, or inside of an abandoned mansion, attacking only those that attempt to destroy it.  Be a kind of cool roleplaying experiencing if the players aren’t all hobo-murdery.
6. Kid has been bullied all his life in the village.  Stumbles upon the book of the dead from some “helpful” trader (actually an Annis Hag.  These fuckers love to corrupt children).  Reads book.  Wants revenge.  Digs up bodies.  Creates flameskulls as revenge.  Murders bullies.  Gets revenge. Becomes evil necromancer. Heroes find out it’s the little kid killing people and terrorizing the village.  What do?