Building a big bad

[Ownagepants]

Ok so my characters are about to possibly fight a rather powerful character it's a lord of outer night but due to him having been trapped for decades he's severely weakened . So I need help building him that he would be a challenging fight for 4 characters at chest deep without also being so strong that nothing short of magic Dm powers could save them .

[Sanctaphrax]

An appropriate challenge for your players depends on what your characters can do. Not all Chest Deep characters are created equal. What are their stats?

That being said, one principle holds true for all boss monsters. You've got to account for the massive action advantage the players have. The ability to attack zones or otherwise harm multiple characters is a must; if the monster has a chance of winning with single-target attacks, it probably hits too hard.

[Mr. Death]

For balancing, yeah, the effective stats are much more important than refresh spent. A character with one refresh spent and an attack roll of 6 is going to be more dangerous than a character with 8 refresh spent, but an attack roll of only 2.

Look at your PCs effective combat rolls ("effective" here meaning factor in any applicable stunts and powers). If you want this to be a real disadvantage for them, put the villain's stats at 2 higher than that.

Does the Big Bad know about the PCs? If so, have him be prepared. If there's a huge magic blaster on the team, give the Big Bad an item that protects against magic.

Maybe he has some power that they haven't accounted for. They're prepared for a brawl with a Red Court, but maybe not for a Red Court with illusion powers, for instance.

Another way to mitigate the action advantage Sanctaphrax mentioned is to give him some goons. Or create another concurrent situation the PCs have to deal with so they're not all just focused on mass damaging him.

[Ownagepants]

maybe I could say he was sealed away with some of his more loyal followers or perhaps
Have the building start to collapse but give them ways to keep it up so not all of them can
Attack at once .

[narphoenix]

maybe I could say he was sealed away with some of his more loyal followers or perhaps
Have the building start to collapse but give them ways to keep it up so not all of them can
Attack at once .

I can't say that I strongly recommend that last bit. Players have enough ingenuity, generally speaking, that if you're counting on that they're gonna fuck it up for you.

[Mr. Death]

maybe I could say he was sealed away with some of his more loyal followers or perhaps

How's he being unsealed? Maybe there's a cult that's been working to free him or something.

Have the building start to collapse but give them ways to keep it up so not all of them can
Attack at once .

It's doable, but as noted, could be turned against you if your players are clever with compels.

[Ownagepants]

The artifact they have unlocks it when they enter the prison .

[bobjob]

I used some elements of boss battles from MMOs in a D&D game I ran a few years ago that really went over well with the players and helped even the action advantage of a group of PCs vs One Big Bad. D&D 5e did something right with Lair Actions for their legendary monsters, but what happens if the PCs are not fighting legendary monsters, but simply a named villain? Legendary Monsters often have a Lair that 5e treated as kind of a separate character that could only be defeated when the main monster was defeated (or if you removed the monster from the lair). The Lair always acts on Initiative Count 20, can do one of three different things (usually an attack of some kind, alter the environment in some way, and possibly some kind of a defense). The entire premise is that the lair is under the monster's influence because it's lived there for a long time.

What I ended up doing is giving the named villains two initiative counts and the second initiative count would go away at half hit points. The villain could only either attack or move on it's second initiative count. I usually had them move, which kept the villain from getting boxed in by the players. The villain would change some environmental variable, remove itself from a surround, or use magic to escape to another place. It kept the battles constantly moving and the kept the PCs thinking and engaged.

Now, I'd like to preface my suggestion with the fact that I haven't run or played in DFRPG in a couple of years, so I'm a little rusty on rules. If I were to do something like what I'm suggesting in DFRPG, I would build a custom "Boss" power. It would have it's own stress track and would essentially act like another character in the battle, one that goes off of a set initiative based on the level of "Boss" power that's initially purchased. As the power is used it would tick off a box of stress. Once all the boxes are ticked off, the Boss power would go into hibernation. Before the Boss Power is used up, it would certainly make things very interesting for the PCs and should be enough to make the PCs expend some serious resources. It would, in my opinion, provide for a more epic battle. Minions are also very useful, but sometimes it's even more fitting to have someone who could take on three or four PCs just by himself as a show of power.

If I can find my PDFs, I'll try to write something up just to see what it'll look like.