Is this "Siren" creature balanced?

Question

I just finished playing a 6th level 4 players session and the GM used this creature in a fight with us:

https://www.dandwiki.com/wiki/Siren_(5e_Creature)

Specifically it used the ability:

Sing (5/day). The siren releases a debilitatingly beautiful song. All who can hear it within 120 feet must succeed a Charisma saving throw of DC20 or be stunned. At the end of each of its turns, and each time it takes damage, the target can make another Charisma save. On a success, the condition ends and they become immune to the siren song for the next 24 hours.

We have two CHA based players in the party, so they managed to eventually snap out of it, but me and one other player couldn't even make the saving throw because we have -1 to CHA.

The ability seems insanely overpowered to me, but at the same time without it the Siren would be useless. Is this creature badly designed? What would be the appropriate DC for this throw?

Answer 1

It's very poorly designed, mostly because of the ridiculous save DC. A DC 20 saving throw is suggested for a CR 21 creature. This, combined with automatically hitting the entire party, and slamming on a condition that completely shuts down the character, and having no way for any freed character to help their allies get out of the song, makes this creature far more powerful than its CR suggests and very lame to fight against.

If you survived this encounter at all, probably there was a lot of luck and limited enemy tactics involved. If you add a single one of these Sirens to an appropriate difficult fight, even one with level 10 characters, it will almost certainly end with people dying. And bored and frustrated while dying.

For an example of how to design a creature like this the way the game's intended, you need to look no further than the Harpy, which is D&Ds own take on the myth. (Spoiler: their save DC is only 11, and they are CR 1)

Answer 2

Balanced? Maybe not, but does it matter?

At the end of the day, DMs have a huge amount of leeway in picking AND in using monsters. It's not unheard of to change the monsters on the fly to make the encounter more interesting or faster/slower depending on the the game is going.

While the rating or stats of a monster may seem "too much", it's also very much about how enjoyable was the encounter. Did you have fun? Was the encounter enjoyable? Was it both challenging and interesting? If the answer is yes, then the on-paper 'balance' really isn't that important.