TLDR: I am considering removing a players arm in combat, is this something I should speak to him about before our session?

I've been running a 4 hour campaign weekly for about 8 months now starting at level 3 and the party is now level 8, just for context on how attached the players are to their characters. At the end of our last session a party member was able to use Dimensional Shackles to cuff himself to a Commander of the BBEG in our story, and get an attack off while the enemy was surprised. This initiated a combat but as we were at the end of our session I ended there and will begin the combat next week.

There are two other lieutenants who are incredibly devout to the Commander in the room and my first thought would be to have them try to sever the PC's arm at some point in the combat to free their Commander. I am going back and forth on the idea as it feels kinda cheaty. If I were to go through with it, is it something I should speak to the player with beforehand as it is pretty drastic, our campaign is pretty wild and something like this happened a few months ago where a PC lost their eye and that went over fine without any prior acknowledgement, however losing an arm feels different as the PC is a Ranger in this case. Does anyone have any recommendations on how to proceed? I also would appreciate any input on how to actually remove an arm during a combat. 

Any and all feedback is appreciated!

One issue for me would be, are you going to allow PCs to start calling shots as well? It’s really a can of worms you could open.

Second if this a character that uses both hands (a bow, or dual weild) you’re really messing with the character. There are ways around it, like a regeneration spell or a magical prosthetic, but if that’s the plan, you should think of a way to implement it fairly quickly. 

I’m guessing the shackles were to prevent teleporting away. If that’s the case, you could have the minion aim for the PC’s arm, miss, and lop off the commander’s arm. Then the commander, who is very angry, teleports away, and next time they see him, he’s got a cool magical prosthetic, and a chip on his shoulder. 

5e doesn't have Called Shots by default, and I wouldn't introduce the mechanic post-hoc for this scene. It's very likely that your player would not have used the Dimensional Shackles this way if they'd known it was possible to cut off a character's arm in combat; adding mechanics that might have changed your players' choices after they've already made those choices is more than a jerk move, it prevents your players from feeling confident in their understanding of how the game works.

Here's what you could do: the lieutenants can cut off the Ranger's arm if they kill him. Players already understand that getting killed is bad; it's not a sudden change to how combat already works. Have the lieutenants prioritize the Ranger in the following combat; make their intent to cut off his arm very clear. This creates the feeling that these characters are following a strategy, and adds tension to the situation without changing the mechanics in an unfair way.

I really like your suggestion! I think something along those lines would play very well with my group, thank you for the feedback!

If it is important to campaign that the leader survive, then cut off HIS arm. The henchman can look menacingly at the Ranger then Slash! Cuts off the leader's arm.

While the henchmen are getting away, have the arm become animated and attack the Ranger.

Or have the arm stealth follow the party to keep track of their location and plans.

The arm cut off could work without called shots if the baddies rendered the character unconscious, and then once they were down you could just say "they cut your arm off" and count it as two failed death saves. 

This is an instance where it sounds pretty OK to do something harsh like this, since this is the result of a choice your player made. Your player must have known this strategy carries heavy risk, and might even think it's a cool story beat for their character to overcome the loss of their arm and take revenge on the one who cut it off someday. 

Also, maybe it's not even a given that they do get the arm chopped. Maybe the players win-- that's still up in the air. Another reason why I'm ok with it: you're not just doing it to punish players or railroad them with a cutscene, this whole situation is completely dependent on the players' actions. 

I would say it is ok if it is being done with intent and not just some willy nilly consequence of combat, I mean your player did take a pretty big gamble and as cool and creative that is they probably should have known that it would also make their character vulnerable as well.

That said I probably wouldn't do it as a called action while the player is still up and fighting, I would say only lop off the player's hand if the commander or his lieutenants manage to reduce the player to 0 hit points with a slashing weapon and narrate it as the enemy striking a vital blow severing the players hand freeing the commander and the player falling unconscious on death's door.

But if the Commander needs to make an escape and they can't bring the player down they could always cut off their own hand, always a cool moment in any story where in a moment of desperate determination the character has to make a split second decision sacrificing one of their limbs to achieve a greater goal.  

There's another alternative: attacking the Shackles themselves and destroying/splitting them.  Typically, once a magic item is destroyed its effects are nullified so if the lieutenant is strong enough then it should be possible to destroy the shackles with an attack.  Though this gets back to the issue of 5E's lack of rules for damaging items.

Quote from 6thLyranGuard >>

There's another alternative: attacking the Shackles themselves and destroying/splitting them.  Typically, once a magic item is destroyed its effects are nullified so if the lieutenant is strong enough then it should be possible to destroy the shackles with an attack.  Though this gets back to the issue of 5E's lack of rules for damaging items.

Well the item description does say that you can break free of the shackles with a DC30 athletics check once every 30 days but without knowing what sort of stats the commander is packing it generally isn't the type of roll that is meant to be beaten by most creatures.

But one thing we are failing to consider is how are the rules regarding the dimensional shackles being interpreted? do Both shackles need to be attached to a creature before the magic works or just one? And thus would cutting off the player's hand allow the commander to escape since one of the commander's hands is still stuck in a shackle? Or are we using a completely different set of rules for the dimensional shackles where in order for for a creature to be effected by them they need to be shackled to the user keeping them on the same plane of existence?

If the commander can't beat the DC30 athletics check I really don't think it would be fair to allow him to attack the shackles themselves and destroy them, obviously these things aren't meant to be easy to get out of or destroy so just letting the commander destroy them would feel like a major cop out and a bit of a let down.

Dispel magic could work depending on your interpretation, not the most interesting option but an option if the commander or one of their lieutenants is packing it in their spellbook.

But personally I like the idea of somebody losing their hand, sounds like a good memorable way to end the encounter and the more I look at it with the rules around the dimensional shackles (even if we allow it to work with only 1 hand shackled) I think it is really the commander that needs to lose his hand here, that is if you want him to escape of course.

If it was part of session zero and they knew what they were getting into, that is one thing. Otherwsise this might be too much for the players in the middle of a fight. 

And as previously stated, if the DM can do this so can the players ( usualy the other way around).