New DM, need help with a Monster

[Jimathee]

To begin, I am a new DM, and still a relatively new player. My buddy, who normally DM's is getting a little tired, and we are using this as an opportunity to get me feet wet, so to speak.

Anyways, I would like to use a Mindflayer dragon as the end boss of my little side story. However, the party's characters are all still too low for a standard, out of the box Mindflayer dragon (with a CR range of 9-15ish). I was thinking of throwing in some allies to help balance the fight, but what suggestions to you all have on how I can balance this encounter, or perhaps lower the threat level of the monster, without my players just feeling like characters in a story, along for the ride?

Thanks!

[DrKerosene]

Are you talking about the Brainstealer Dragon on DnDBeyond or a Homebrewery site?

Without actually knowing anything, I would arbitrarily say that reducing all the stats by 1-2 each, and reducing the spike damage it can deal (either lesser damage dice, or removing lair action options), should work for the most part. Further reducing the proficiency bonus by a point or two, and the total HP/AC would probably be all the other changes you need to make.

If you’ve done everything, the Attack Bonus and Saves of the creature should be down about about 2-3 points everywhere, it’s damage output has probably gone down by 3-6 points per hit along with being less accurate, and the Party is probably 10-15% more likely to succeed at whatever they want to do. The AC and HP are the only bits that probably need minor or major consideration to get the overall CR to where you want.

[GreyBlack]

I found this video helpful in considering how best to change monster stats. The basic gist of it is to reduce the CR of any abilities by 1-2, reduce the hit dice by an appropriate amount, including their average HP, and then reducing the damage by however much is needed.

https://youtu.be/QgTIGo6zJbs

[Wizard_Lizard]

Makeit have a weakness.
i.e if you hit it in theye with the macguffin, it will be weakened a lot.

[Jimathee]

These suggestions have been super helpful, thanks!