Help with homedrew campaign

[GrinningDemon]

[5e] I'm running my first homebrew one shot. My question is im working on the end boss and im torn on what to put in that isn't too overpowered but challenging just enough. I know I want it to be some kind of fiend, devil, or fey creature. I was thinking shadow demon but not sure. The group so far is
Goblin - arcane trickster
Warfordge - barbarian
Veldaken - articifer
Half orc - barbarian
Half elf - archfey warlock
Half elf/halfling cleric
And they are all 5th level

[Kvess]

My best advice is give your boss minions. A party of 6 will make short work of just about any individual creature, especially if the party wins initiative.

Interesting terrain can also improve a boss fight. Enemies that stand in wide open spaces have a tendency to die quickly, so have them dart behind doors and into hallways. A smart devil or fey makes the players chase them, so they can confront the players on their own terms.

[aglondier]

Fun extra to add. The boss, big scarey title and eldritch powers, is actually a rather innocuous looking intelligent magic item. It has possessed the critter who is your big bad guy, and will fight to the end. Then the party loots the corpse divvys out the loot and ride off into the sunset...

...then the next campaign features a big bad with the same name and powers, but also with a handful of new tricks...behold, it is the pc from the last campaign who ended up with the item...each time it learns new abilities and gains new powers from the creature it possesses...

[HumanFighter]

Fun extra to add. The boss, big scarey title and eldritch powers, is actually a rather innocuous looking intelligent magic item. It has possessed the critter who is your big bad guy, and will fight to the end. Then the party loots the corpse divvys out the loot and ride off into the sunset...

...then the next campaign features a big bad with the same name and powers, but also with a handful of new tricks...behold, it is the pc from the last campaign who ended up with the item...each time it learns new abilities and gains new powers from the creature it possesses...

Oh that is an excellent idea

[martixy]

This is a chance to learn something not taught in any D&D book.

You have a party of 6. You are planning a boss fight.

The action economy wants to have a word with you.

If you put a single creature vs 6 players, they will curbstomp it. If you make it tough enough to stand its ground, it will likely curbstomp them. You need minions or something to soak actions from ur players. They can even be 1HP, as long as they present a threat to your players, forcing them to divert actions, they've done their job.