I am new to playing 5e, and my DM is also new to DMing 5e (although they have extensive prior experience as a 3.5e DM). We are interested in using the Sidekick rules presented in Tasha's Cauldron of Everything to round out a party since I am the only player. My character is a Fiend Patron Warlock whose Patron is an archdevil. The most thematic starting Sidekick I can think of is a low-level devil sent by my Patron both as a boon and also to tempt me into more evil deeds in order to hasten my corruption. This keeps the spotlight on my warlock and hooks into my existing backstory quite nicely.
I searched dndbeyond and only found 2 devils with CR between 0 and 1/2: The Lemure and the Nupperibo. Both seem incredibly stupid and uncharismatic, and thus unlikely to be much of a corrupting influence at all (instead it just seems like a body in a fight, and a liability in any civilized setting).
Expanding the search to CR 1 helped me to find the Imp, which is just barely illegal, but thematically perfect. 14 CHA and 11 INT make it easy to RP as a slow, steady corrupting influence, while Shapechanger helps it from becoming a huge liability when I need to spend time in civilized areas.
I think the best option is to tweak the imp until it is CR 1/2, and then give it sidekick levels from there, but I'm open to frame challenges (especially from anyone who played with UA sidekicks which did allow CR 1, who might know the Imp is fine as is, or that the Imp is especially problematic in ways which will still be present even at CR 1/2).
I plugged the stats into two different CR calculators for the base Imp, then played with options until I got CR 1/2. I was surprised to find that both calculators agree that reducing the AC by 1 is all that is required to bring it down to CR 1/2 (Offensive CR of 1, Defensive CR of 1/8). I tried to follow along with the DMG and it also looks right to me:
Defensive CR:
- Actual Hitpoints of 11, Effective of 22 due to resistances/immunities means it is in the CR 1/8 range)
- reducing actual AC from 13 to 12 means that Effective AC drops from 15 to 14 when factoring Magic Resistance, which means no adjustment from AC since CR<=1 is expected to have 13+-1 AC.
- The base Imp barely falls outside this range, which increases Defensive CR above 1, which is what pulls the entire creature up to CR 1
Offensive CR:
- Damage/Round is 15, which is barely base Offensive CR 2
- Damage is primarily based on Save DC of 11, so Offensive CR gets adjusted downwards by 1
Given that this Sidekick is going to be with me for the duration of the campaign (and thus matters a lot more than any monster in a one-time combat encounter), and that the UA Sidekick rule was deliberately scaled back from 1 to 1/2, I want to be sure I didn't go astray for my first attempt at adjusting Monster CR.
Best Answer
Monster Challenge Rating is not exact science
The very fact that you can change AC by one to change the CR (on paper) shows the limits of the approach. If the AC-1 imp is okay the normal probably is, too. I know that you have to set a limit somewhere, but really there's a continuum and the AC change of one is not a far way on the continuum. Either they're both okay in play, or not, or they're both approximately at the limit.
As a single player you're probably fine
I used some "legal" sidekicks (humanoids with CR 1/4 to 1/2) and they mostly weren't that impressive. If you consider the sidekick as an additional person in the party, it seems you won't be easily unbalancing the encounters. Since you're alone you're already at a disadvantage compared to a diverse party, so it's even less critical.
The imp is offensively skewed.
The imp's offensive CR is a lot better than the defensive. This is a typical problem in monster design which I try to avoid. Now, since the imp is already better on offense, reducing AC is not the solution since that makes the difference worse. I don't know about later levels, but at the beginning levels a sidekick with offensive CR 2 will overshadow player characters since it can easily wreak havoc on appropriate opponents. So, if you want to reduce the CR of the imp, you should probably reduce the damage to the appropriate level to get there, taking 1d6 or 2d6 off the poison damage will do it. Then again, you're the only player, so the overshadowing problem is not really critical if you don't mind your imp doing more damage than you.