The 2nd level spells Phantasmal Force & Suggestion have a lot of uses and abuses - what can their owner-possessor do with someone who succumbs?

Notes:

'RAW / RAI / RAF' --> 'Rules As - Written // Intended // Fun'

PhB = Player's Handbook (5e). DMG = Dungeon Master's Guide (5e)

Summary of 2nd lvl Suggestion spell (RAW / edited):

• Strictly verbal casting ('V,M') - so no handwaving - only words give away the fact that anyone cast a spell. It is possible that, if your request is reasonable enough, this is one of the few spells you can cast completely in public and no one ever would know (or find out). Also, unlike Charm, the target may not even know that magic was involved, even after the spell ends. They may feel they acted out of character, but, whatever they did would appear forever after as their choice.
• This spell is cast in a 'sentence or two'. If you read any old philosophy texts you know that this could give you about half of an hour talk-time.
• Must sound 'reasonable'. What is that? Direct quote from the 5e spell (PHB): 'give your horse to the next beggar you meet!' is reasonable, apparently. Conversely, self harm is unreasonable, according to this text. The spell also ends if the caster (or anyone who is believed to be a friend of the caster) hurts them. We must assume that total strangers are free to stab this charmed target at leisure without any effect on this spell's duration. This also suggests that the victim of this spell is aware of the caster's social politics - even though they are totally unaware they are under a spell in the first place (???).
• Concentration - maximum eight hours. Like a supervisor for roadwork, you can get someone to work for you for an entire shift so long as you are watching over them whilst eating donuts (actually, the caster doesn't even have to be there. 'Concentration' means that you cannot cast another concentration-spells in this enchanting time, a new concentration spell breaks the fist one.

Summary of Phantasmal Force (RAW / edited):

• You 'craft the illusion' that only they see that is equal to or less than 10' on any side. Thus, a Gelatinous Cube fits perfectly.
• You gesture in their general direction ('V,S,M') whilst mumbling weird stuff... and THEN crazy stuff happens. As such, everyone (target & other observers) may suspect shenanigans.
• The target may well think they are hurt by any element but this spell really does 1d6 psychic damage (per round / one minute max. = ten rounds). DMs have to be very clever describing this to Players without them catching on that something is amiss.

Points: Similarities & Dissimilarities

• Suggestion is the standard wisdom-save while Phantasmal Force is one of the rare intelligence-saves. Technically, the first is a 'charm'-type spell (so-called 'enchantment') whilst the latter is 'illusion' - they will effect / affect different targets with different results. Both spells require concentration - so one caster cannot easily use both simultaneously, thank Glittergold.
• Both seem to have near-endless possibilities for unmitigated target control. This is the focus of this discussion below. Please comment if, for any reason, you feel uncomfortable reading this. As a DM these 2nd level spells may seem a bit... powerful. If you feel some of the suggested uses below are a bit wet & wild, you are not alone.

Discussion: Possible Uses & Abuses

Phantasmal Force: A lot fits into a 10' cube. Even with slight creativity one can do so much more than 'distract target for ten rounds doing 35 hit points damage total'. Virtually all the conditions listed in the back of the Player's Handbookare possible with one phantasmic casting. For example, trussing someone up with imaginary Evard-like tentacles is easily accomplished, allowing anyone to then tie up this target with real bindings. Yet why stop there? Why not put the target in mist? Vision is obscured. Try poisonous mist? Poisoned, giving disadvantage on all attack rolls / ability checks. How about black poisonous smoke? Now the target would be blinded, vision-obscured, poisoned whilst suffering progressive levels of exhaustion from asphyxiation. For fun you can throw in the tentacles and bonus 35 damage - all transpire with only failing one save. As a DM this may seem to be too much - though players might disagree... until you try this exact trick on them. Then suddenly harsh words like 'railroading' crop up, everyone will get emotional and there goes the game night. This doesn't even mention summoning up phantasms of ghasts or shadows that have even more powerful effects giving imagined paralysis or imagined loss of strength.

Suggestion: Remember the example listed in the Player's Handbook?: the knight gives his horse to the next beggar he meets. Right? Right. If any of the readers have ever owned a dog they would already see the concern here. Under what circumstance would you give away your best friend? Let us up the ante: let your dog be your means of transport, your travelling alarm system, a trusted combat ally, your means of livelihood and your status symbol well above 99% of the population. As a knight this horse is your everything - all on four legs. Under what circumstances would you give that to a guy that would probably chop it up for meat within the hour? This is the part that slays me: you are told to do this and you think about it the whole way, possibly up to eight hours, and you never reconsider how terrible a decision this is. You may feel that this is a 'fluff' spell and allow it to work as RAW. Others may point out that this spell is supposed to work in 'reasonable' circumstances - this simply does not fit that RAI.

Possible Solutions for YOUR world

The RAI of 'reasonable' can really work for you and your players. These are only 2nd level spells, they are not supposed to be far more powerful than Polymorph or even Otto's Irresistible Dance. Here are Suggestions as to what RAF-rules you may put in ad-hoc, both for your players as well as your NPCs using these in your world:

• any spell aught to produce one restraining-style effect at a time. As Phantasmal Force is a local-only concentration spell, the DM may allow the caster to change the effect. Example: The caster produces toxic billowing black smoke (as above). One round it causes blindness, the next round the target feels asphyxiation from the transparent gas (but can see through it just fine), the next round could be something else entirely - as long as it reasonably relates to the summoned phantasm (in this case, smoke).
• saving throws aught to function as makes sense for the target. Imagine your target is being attacked by phantasms of Evard's Black Tentacles - they would get a saving throw, with strength or dexterity, each round (INSTEAD of an intelligence save). Why? As it is the target's nightmare they would defeat this with the limits of their faith. An ogre would simply not believe that he or she would be bound by the silly tentacles, no matter how unwise that little giant may be - whereas the thin priest-magi elf would feel they would be a toy amid those tentacles, no matter how brilliant or well-studied they became. 
• magical effects would pend the reliability of the target audience. For example, a phantasm of a Shadow would only drain strength if the target had a history of having been successfully attacked by such an undead. That said, if this was the targets first exposure to such a monster, their saving throw against fear would be at a disadvantage. This makes the rules reasonable, interesting and fun.

How strong to make your Suggestions - also here, a discussion of 'Geas-Quest' spell.

• Use of Suggestion aught to lie somewhere safely between Charm and well below Geas / Quest. Player characters often point out that the damage to the Geas / Quest does not count because players heal everything overnight - as such, the punitive effects of this spell only are a concern if they happen just before combat. This is not remotely RAI or even RAF. As a DM i would gladly put in levels of exhaustion each day the characters went off the track of their given quest - and this feeling of pending doom would only subside as the character got on track. Suggestion takes a much softer route. If you would enchant a knight to give his horse to the next beggar he sees, he would agree and go off looking for this beggar - but the spell would break in the process of the handover as this knight would change his mind. Similarly, if you managed to convince a dragon to hand over their hoard, they would gladly tell you where to find every gem, trophy or coin in the entire hoard up to the break of spell-concentration. That said, the moment a player would accept so much as a copper piece they take their life in their hands. This means Suggestion functions a lot like a dear friend asking you to play along. One writer suggested that it should be as 'that night you got really drunk and did wild & crazy things'. "Remember that Friday we all drank to the point of not remembering much? Yea! We got it all on film and it was totally nuts!!!"

Suggestion can still be a very powerful spell. You can Suggest even chaotic and hostile creatures tell the truth because they have nothing to be ashamed of (they aught to feel proud of who they are and what they have done!). You could ask a creature to teach you how to be a trusted life-long friend so that the end of the spell allows you to have an excellent henchman (and this would work, depending on how your personalities match). One can also use this spell to get anything which costs the target reasonable effort. Just imagine: dragons attacking those annoying small hobgoblin armies - OR - suddenly friendly wizards casting many spells (that require cheap components) - OR - otherwise nasty but dim-witted giants gladly doing hard manual labour for eight hours... the list goes on and on. As you can cast it so often and it leaves no trace, this spell can potentially be far more powerful than anything in the book, Wish included.

Questions you as a DM may still have

• How do you reward creativity? A player may say to a warrior 'defend us from the dragon for a round or two whilst we make our escape!' and it would be decided (on save-failure) that this counts as 'reasonable'... and a very creative use of the spell at that! Then the player starts going around town sending the entire guard sentry off to the dragon's lair: i.e. the player says to one or more each day 'go for a walk around that area, into that cave for sure... it is so pretty this time of year', eventually launching an entire standing army one-by-one to die in the hands of a dragon. Irksome perhaps? How do you draw the line between a single-use 'fun' cast of this spell and a mechanically abusive routine usage that is game-breaking in the long run? You may have to introduce NPCs that are specifically savvy to mind-altering magicks with most of your common-folk.
• How do you stop the abuse of phantasms in more peaceful situations? It would be possible to have a person witness virtually anything (examples: see the discovery of any powerful magic item - OR - witness two people saying &/or doing dastardly things - OR - watch a theft, 'overhear / oversee strange events... the list is endless. Targets of this spell would never be able to discern reality nor would anyone be able to convince them otherwise. They saw it with their own eyes! It looked, smelt, felt and sounded SO real! Any intelligent person (say... a bard... or dare i say a mage?) could easily cause key people to witness even simple conversations that would cause destruction of entire kingdoms overnight. This one is hard to remedy as the passing of Phantasmal Force without a trace to be found by Detect Magic. Still, this could be a very interesting twist on the 'evil Vizier Jafar vs. Sultan' trope: instead of Jafar using direct Charm mind control he would have the king witness certain things at key times.
• There is a MASSIVE discussion on the whole 'is any mind-compulsion a form of rape?'... and we do not go there. Follow the lead from various Disney movies like Aladdin - where not even a Wish from Robbin Williams could change how people feel. You may choose to go in a different direction in your game, but be advised to tread carefully. Game rules, rulings & R.A.W. leaves this open, i leave this closed.

You may also feel that these two spells are very different in theme with radically different issues, perhaps i should have simply made two separate posts. I feel that the theme of 'balancing mind control' is something that is somewhat overlooked in most combat-heavy games, which is why i also mentioned Geas / Quest in this post. Speak your mind! In fact, i Suggest you write down your experiences with these spells and voice (well, 'write down') anything that comes to mind in the comments below. Also, be good to yourself and your friends. Eat oats and broccoli - but not at the same time. Go to bed at a reasonable hour... and as you fall asleep... realize just how much you want to share this article with your friends... yes you do... yessssss....