Hi, not sure this is in the right place, or if it should be in homebrew?

I am looking to create a statblock for a boss encounter from an old character who went evil. Using the create a monster guidelines, kinda helps with general hit points, AC etc but how to balance abilities, legendary actions etc I’m a bit vague on? I want this boss to feel like the old characters so will have some of the old abilities but I’m not sure if I just supe up a character hit point and damage output and tag this onto a dnd character sheet? Also they had a couple old magic items which I’d consider letting the players get afterwards but how would you justify a magic item doing loads of damage when weilded by the boss but only being a magic spear when the party get it?

for context, in a previous campaign one of my old characters became evil and left the party, his story was never resolved. Now I’m DMing and after talking with my old DM we discussed giving them a send off. This guy was a Monk who fell to Leviticus in Icewind dale. In my head he’d have become a warlock with leviticus as his patron. So as a character he’d be a monk/warlock multiclass. The party in my currently game are level 5 and 6 members. Rough guidance is a CR11/12 monster should be deadly? So should I use the guidance for a CR12 monster stat block and reflavour the attacks as monk abilities? How have other DMs created monsters with character levels?

The party in my currently game are level 5 and 6 members. Rough guidance is a CR11/12 monster should be deadly?

This question can't be answered because you didn't specify the number of players. For a standard party of four 5th or 6th level players, a CR 11 or 12 boss can be a tad too much. 7200 or 8400 XP is slightly above half of the daily budget for them. If you intend on having them fight the boss at the end of an adventuring day, you risk a TPK. If you intend the boss to be the only encounter of the day, then I can't really help you because the game wasn't built with that in mind. The math doesn't take that into account when determining how balanced an encounter is. Personally, I try never going above twice the treshold for a hard encounter when making a boss fight, which in this case is 6000 or 7200 XP, or 1600 XP above the deadly treshold in both cases.

So should I use the guidance for a CR12 monster stat block and reflavour the attacks as monk abilities? How have other DMs created monsters with character levels?

XGE has a Quick Matchups table that compares player levels with challenge ratings. The monster equivalent of a 5th or 6th level player is CR 2 in both cases. Since such matchups only create medium and hard encounters, it's not a fair equivalence. Although the odds are stacked in favor of player levels, it does mean that monsters created with player levels should be closer to deadly encounters when using the table as a conversion tool. Bear in mind however that the game wasn't made for PvP, so you will need to test your creation with a CR Calculator to make sure it's balanced. If it's not, then you'll have to make adjustments. According to the table, you should start with a 20th level character to challenge your players. There's a good chance you will need to decrease its damage and increase its hit points. Player characters are in fact glass cannons when compared to monsters.

Hi, thanks for the reply. This is a mistake in how I wrote the description. I meant the party is level 5 and there’s 6 party members. Sorry, just realised it reads that the party is a mix of level 5/6. The fight may be the only one, not sure. With 6 players fighting can bog down as such I tend to have fewer more dangerous fights rather than the recommended number per adventure day as some bad dice can mean the party takes forever to kill easy enemies (something that happened when face a AC13 monster and unfortunately having established the AC, the party proceeded to not roll above a 12 for multiple turns). Technically the boss is more a side quest level boss so maybe CR11/12 is a bit high. Maybe 9/10 would work better.

As to the second point I completely agree, hence why I wasn’t sure if it was just a case of buffing health. The recommendation for damage per turn would probably kill a player each turn which I’d rather avoid. 

thanks for the link to a CR calculator, I was basing stats off existing monsters, hopefully this will help.

I meant the party is level 5 and there’s 6 party members.

In that case, the boss fight becomes much easier. A CR 12 monster against a team of six 5th level players counts as a medium encounter. At this point, you can't call that a boss fight. Unless you give the boss some minions, it won't stand a chance against such a huge party of adventurers. Case in point, a CR 15 monster is almost a deadly encounter for them, with only 100 XP below the deadly treshold. Do you use the Encounter Builder?

With 6 players fighting can bog down as such I tend to have fewer more dangerous fights rather than the recommended number per adventure day as some bad dice can mean the party takes forever to kill easy enemies (something that happened when face a AC13 monster and unfortunately having established the AC, the party proceeded to not roll above a 12 for multiple turns).

That shouldn't be an issue. It doesn't make sense for weak monsters to continue fighting to the death when it's pretty obvious the odds are stacked against them. They should run away or, in the case of sapient creatures, surrender and beg for mercy. For example, I once had a party of four 1st level players come across three goblins blocking the road and extorting money from travelers. They had a choice between handing over their coins or fight. They chose to fight, but two more goblins were hiding in ambush, so what seemed to be a hard encounter proved to be a very deadly one. Nevertheless, my players took out three of the goblins without suffering a single casualty. The remaining two tried to run away, but my players pursued them and took out one of them. Fearing for his life, the last one took a chance and asked from his hiding spot if he could surrender. My players agreed and the goblin was spared. They received 250 XP for defeating all five goblins.

Had the two goblins escaped, my players still would have received the XP in full. Defeat doesn't have to equal death or unconsciousness. Two of the goblins perished, one was instantly killed and the other failed his death saving throws. The unconscious goblins were tied up and the conscious one negotiated for his liberty. He told the players that an ogre bullied a tribe of goblins into submission and forced them to steal shiny things for him. The goblin promised to stop stealing from travelers if the players kill the ogre. My players accepted and let the goblin guide them to his hideout. He actually kept his promise after the players liberated the goblins from the ogre. As you can see, realism creates new role playing opportunities in addition to not making combat a mathematical chore.

Technically the boss is more a side quest level boss so maybe CR11/12 is a bit high. Maybe 9/10 would work better.

You're going to bore your players if you do that. A CR 9 "boss" in this case would count as an easy encounter. You really need to use a program that does the math for you instead of relying on your gut feeling when creating balanced encounters.

Quote from M3theston >>

As to the second point I completely agree, hence why I wasn’t sure if it was just a case of buffing health. The recommendation for damage per turn would probably kill a player each turn which I’d rather avoid.

 Unfortunately, that's pretty much a requirement when you're dealing with 6 PCs. If you want the PCs feel like they've been in a real fight, you want the fight to last something like three rounds, during which the PCs take half their hit points in damage. For anything single target, that translates to one downed PC per round.

Except that's not even good enough, because something that merely does enough damage to drop one PC a round (CR 12-ish) probably won't live for three rounds, unless you give it max hit points or something similar.

My general rule of thumb is that by the end of the first turn, either at least one PC is down, or the majority of the party has been hit by area damage that halves their hit points.

Thanks for the advice. Yeah usually I do use the encounter builder but since I’m trying to homebrew the monster starblock and dnd beyond just has you type that in I’m mot sure if I’m creating an equivalent CR9 monster or CR12? I was planning on legendary actions to balance action economy a bit, although minions is a good idea. Yeah 6 players has trivialised some encounters, I’ve had to beef up quite a bit from modules in the past to challenge the players, usually I’d use existing monsters but this time I’m trying something unique. I’m just worried about breaking it and either having something that’s a pushover of too strong, so CR of the boss is where I’m aiming with the starblock.

Interesting point, I’m always worried about overkill, and taking players out the fight. When you’re saying about dealing enough to kill a player would focus on a character or spread the equivalent damage around so it doesn’t feel targeted at a player?

Quote from M3theston >>

Interesting point, I’m always worried about overkill, and taking players out the fight. When you’re saying about dealing enough to kill a player would focus on a character or spread the equivalent damage around so it doesn’t feel targeted at a player?

 Dropping a PC to zero doesn't take them out of the fight; that's what heals are for. Unless you're using house rules that make being reduced to zero unusually punishing (or have a party that's very light on healing), a challenging fight for six PCs means 2-4 of them probably drop to zero at some point in the fight.