I'm planning on running this guy as a boss at the end of a tomb filled with undead. Any tips?

GREEN FLAME! 

After our party defeated one of these for the first time, my character said, "These are made from the skulls of ancient wizards? Cool" and proceeded to Mend the broken skull and stick it in her pack. Hilarity ensued next session, in the middle of another battle, when my pack started hissing.

My DM gave me this as a familliar after we defeated it. I rolled a nat 20 on arcana and learnt how to reprogram it for my needs but the world doesnt really appreciate me walking around with a skull following me xd

Ik Ik imma bit late on this like a lot late on this but good job I would do that too but another group I am in already did it like there was another dm for that but story but good job it’s always good to have good moments in a story

Quote from xander101 >>

Just remember that you have to be the only one making a Dex save in order to make use of that feature (I know, it's a really stupid rule, but regardless, check it for yourself, it's there)

Ran these in CoS level 6 party took care of 3 of them but not without taking a good amount of damage.

FLAME!

i am so using this guy as a boss for my player (lv2-4 guys), JA JA JA!

Quote from Drake_Fairwater >>

I am currently using this creature as a magic advisor to the party. I changed some flavor about it's abilities and alignment. So far the party seems to enjoy having a skull as a DMPC that can identify magical items, recall information about historic events and give advice on how to overcome problems when they get stuck. I limit it's combat abilities and strip away the Fire Ray so that it cannot be used as a weapon. I changed the spell load out to be more utility spells such as Identify, Message and Mage Hand.

 

I just find it a fun and interesting "item" for the players to use if there isn't already a resident spell caster in the group.

 Actually, I recommend not changing alignment. Having an evil creature in a group of good creatures is always a bit funny.

My PCs fought the one in Wave Echo Cave until it self-destructed with Fireball. Then they went and long-rested at the other end of the map.

On their way back up, who should greet them but the Flameskull again! They beat on it some more, so it blew itself up again. This time, the Paladin thought he’d outsmart it by leaving only half its fragments on the ground *and scooping half its fragments into his backpack.*

Then they long-rested again. I flipped a coin. It didn’t come up heads, so the insane, malformed thing woke up where it died instead of in a backpack. It showed up at their last battle later to just murder everybody it could and steal back its missing pieces.

As a finisher, it exploded AGAIN. The party mopped up the baddies who were left, got their share of the mine’s profits in writing from Rockseeker, and called it a day.

The followup adventure was at a charity concert partly to install a memorial to the miner recently found dead with 3rd-degree burns and bits of skull embedded in him.

Next one is gonna be a skull hunt.

Getting a lot of mileage out of this little guy.

Freaky

I created a homebrew necromancy spell that allows you to create your own flameskull.  Here's the link to it:  Create Flameskull.  I just copied animate dead but changed all the stuff about zombies and skeletons to flameskulls.  If you have a player that is playing a Necromancy Wizard, Undead Warlock, or Death Cleric, it is recommended that you allow this spell into the game.  Maybe give it a try, and if you find it too overpowered, you don't have to use it.       

im about to run a session with this thing and im definetly gonna use a description like this, thanks

im doing this aswell, one of my players got hit with the fire ray and is on 1 hit point... and i have 7 players

I had my first character, a warlock, die trying to solo this ****er after he regenerated while the party was sleeping during lost mines of Phandelver. it was a good time, but he's now become a meme in our dnd-group.

On a scale of 1 to Oh Snap, how evil is this: The party encounters the flameskull on top of an Orthanc style tower, orbiting around the top as a sentry, the top of the tower is 25x25 and there are tall stone projections that interrupt line of sight. Since the party has strong ranged abilities, the skull will bob and weave, keeping distance, hit and run style. Fireball will be unavoidable unless they want to jump, swooping into range to for Fire Ray, keeping Blur up and eventually dropping a Fireball. Should be a chore. 

Just resistance to necrotic damage. Why not inmunity? As every undead should have...