Hi everyone,

I need help deciding on the 6th level Mystic Arcanum spell. I understand that there are many good options and I thought about them all, but it went down to deciding between Mental Prison and Summon Fiend. I really wanted something that includes nice damage and Mental Prison not only offers that but is also a great way to keep an enemy out of battle for a long time as long as I maintain concentration. However, damage output of the fiend could be similar or even higher (depending on how long it survives and again me maintaining concentration) and it has also uses outside of combat and also is really cool thematically, especially for me being a Fiendlock with the Pact of Chain liking summoning in general. The only thing that really turns me off in case of Summon Fiend is how little HP those fiends have, with max of 60 in case of Yugoloth and only 40 for Devil. Thank you for your opinions!

If your Fiendlock is big into summoning creatures, then Summon Fiend does seem pretty on-brand. Probably the simplest comparison to make is to Summon Greater Demon. Your Fiendish spirit won't have as many HP as something like a Barlgura, but it'll have higher AC, and it can't break free from your control. Plus, when it comes to the Devil form specifically, you're trading HP for flying and a ranged attack, plus the explosion when they die. Damage-wise I haven't done the math, but the damage output of the Fiendish Spirit looks pretty solid in comparison. My personal taste would probably lean more towards Mental Prison, I found with my Hexblade that spells forcing Int saves can be lots of fun(Enemies Abound cast from a distance while hiding can be GLORIOUS) :) But go with what feels right for your Warlock, they're both solid spells.

Thanks for the tips @jd2319, what makes my decision harder is also the fact that I already have a good psychic damage spell (synaptic static which is also INT save and an AoE) so that should make me prefer Summon Fiend, but I also have Summon Greater Demon spell :D That said, maybe I should just go for Mental Prison since I already got Summon Greater Demon and plan to keep it, but Summon Fiend is naturally much safer.

I've been eyeballing that list too, because it's coming up next level. My personal favorite is Tasha's Otherworldly Guise, but between yours I think Summon Fiend could be more frequently useful. It's probably not the bomb you drop on the big bad, but it'll help you knock through his minibosses without the party burning too many spell slots. The HP is kinda low, but the AC is 18, and I like that you get to make the strategic choice between flying range fighter, exploding melee fighter, or bamfing melee fighter. Plus there's no "This demon will try to kill you too," clause, and whatever you pick will probably do at least some damage in it's three attack rolls, versus the potential single intelligence saving throw that can negate Mental Prison. I think Summon Fiend will just end up being more versatile.

Out of those two I'd go with mental prison. You can take summon aberration with your normal spells known and its probably a better spell even if you wont get the 3rd attack. That being said since mental prison does not work on enemies immune to charm it depends on how often you expect to bump into creatures like that.  It is an incredibly common immunity.

I think the top spell is probably mass suggestion though. long duration, no concentration, it hits a group. You can end fights before they start, and it is versatile.

Thanks for yo opinion @Rockwall4314, I was thinking in a similar way so that is +1 for the Summon Fiend. I also like the sound of Tasha's Otherworldly Guise but I think that is more suitable for a Bladelock or if I could pick more 6th level spells I would consider it. 

Ouch, I somehow completely overlooked that Mental Prison does not work against enemies immune to charm and in my current campaign we have already encountered a few of such enemies. Dang it, that kinda turned me off completely from Mental Prison :D I can see many people liking and recommending Mass Suggestion but I am that type of a character who likes actual combat and defeating enemies rather than preventing combat. Of course sometimes the latter is better, but I think I can convince our sorcerer to pick the Mass Suggestion. Thanks for the insight @MyDudeicus!

For consideration.... 

Create Undead isn't exactly a summons, and requires some preplanning. It does, however, give you 3 "pets" under your control, without concentration, for a full 24hrs. Their damage isn't amazing individually. But there are 3, and there is that Paralyzed effect. As a tie-in with the FiendLock, they're supposedly powered by Orcus. So... yeah, that's a stretch. But it's there in the flavor text for a Ghoul. 

Why choose this for an Arcanum? Well, unlike other casters, you'll only ever have a single 6th level spell. This one gives you a solid way to make use of it every single day. If you have Ghouls already, you recast it to maintain control. If you're down one or two Ghouls, you command the remaining to go fetch you some new bodies, and then recast it. If you have none, then find some dead guys... 

From an efficient use of a feature perspective, this would at least ensure that you get to use your Arcanum every day. Few spells can grant that.  

Speaking of... if you were to choose this route, and then also make it to 13 for your 7th Arcanum... Finger of Death. Yeah, I know it's not the best spell in the game. In fact, it's kind of weak for a level 7 spell. But, you can slowly build a large bunch of zombies that are permanently under your control. In a campaign I"m in right now, we have a NecroWiz who's been doing this. At first, I thought it was a real waste of a high level slot. But he just keeps grabbing a new one every day. I think he has 5 or 6 now. He has them follow us each around and go first in any situation there might be a trap. They carry our stuff. They block opponents (wall of flesh spell). They're actually kind of useful to have around. 

@Christian_Thomas thanks for the tip!

Don‘t forget that with Tasha‘s you will be able to swap your 6th spell level Mystic Arcanum when you reach 12th level Warlock! It‘s like a free shot now. 

@Haudraufundschluss thanks for the reminder!