Providing a Shield Guardian

Hey folks, my players are about to encounter a boss fight where a Shield Guardian and its amulet are involved (part of the Phandelver and Below for those who know).

I kind of expect my players to go for it once they realize what it does. Has anyone run into issues with one of these "joining" the party? They guys will have hit level 7 after that fight, so it being CR 7 should be roughly ok, although still pretty formidable in general for a bit. 

Do you typically leave it fully under the control of the amulet holder? How is it controlled? The encounter stat block has the boss use his bonus action, for which the guardian is an eligible target to be forced to operate, though it can also control other creatures. 

I guess eventually it will lose in efficiency and be left behind as it won't be leveling up alongside them.

Anyway, i was curious if anyone had suggestions on this thing.
Thanks.

A shield guardian is a powerful ally for a level 7 party's character wearring it's Master's Amulet, having full control of it and can command it to attack its enemies or to guard the wielder against attack. As long as the guardian and its amulet are on the same plane of existence, the amulet's wearer can telepathically call the guardian to travel to it, and the guardian knows the distance and direction to the amulet. If the guardian is within 60 feet of the amulet's wearer, half of any damage the wearer takes (rounded up) is transferred to the guardian.

While iy doesn't level up, it nonetheless remain useful during an entire career if not destroyed. 

It's big and clunky though, thus also making the party it travel with not easily go unoticed if you know what i mean.

If you’re worried about it, you can always say the amulet was destroyed in the fighting. Or just have the amulet’s owner smash it out of spite. 
Or just make sure the guardian is destroyed. It’s going to be taking half of the controller’s damage, plus its own damage. I always thought of it as a damage sink so the controller doesn’t die too quickly. 

Well in addition to buffing the amulet's wearer, it can also Spell Storing to cast later when a predefined situation arises, which can lead to all kind of interesting possibilities.

Quote from Plaguescarred >>

Well in addition to buffing the amulet's wearer, it can also Spell Storing to cast later when a predefined situation arises, which can lead to all kind of interesting possibilities.

 Very true. A nice dimension door escape hatch. 

Oh, it's placement is 100% cuz they otherwise made that fight against a single psionic goblin. While a decently strong stat block on its own, there's no way Ruxithid would survive for long on a 1 v 3 fight (the module literally has a "if this is going to fast, throw some psionic goblin reinforcements" note).

I do realize the size of the guardian can soft lock it out of some areas. It's size and weight would also not make it eligible to ride on a wagon, so it would slow down travel pretty significantly.

Despite all that, I'm kind of expecting my players to aim for it anyway. Making the amulet crack could be a thing so they need to repair/replace it.

My biggest question is: how is it handled in a round? What's the action economy involved? Is it a "pet" that can be freely controlled at no action/bonus action requirement as long as you wear the amulet? It does have a slow self-repair, but I imagine repairing it otherwise requires something like Mending. Kind of odd they didn't list that on either the amulet or guardian page.

Actually, idk why I didn't think of this sooner, but I'll treat is as a big bulky "Primal Companion". Sure, it won't have the (rarely used) versatility of a Ranger's pet, but with how tough and strong it is, requiring a Bonus Action to command seems like the absolute minimum for combat balance instead of just magically giving them something that's tougher than the Paladin. A Bonus Action also means they will have to ponder if it's worth not smiting this turn, or the Rogue missing his Cunning Action for that turn. I mean the Sorcerer might still get most of the uptime there...

While you are free to treat it as a Primal Companion, RAW the shield guardian  is an NPC monster that a player character can freely command and which can take various action or reaction as normal.

RAW, there’s no action economy cost. As you said, it’s there to make the fight more even. Adding in an economy cost will be a nerf to the bad guys. 
FYI, there’s a shield guardian amulet item in RotFM. It says making one takes 1 week and 1,000 gp in components, just as a point of reference. Also it has a 10 AC and 10 hp.

Quote from Xalthu >>

RAW, there’s no action economy cost. As you said, it’s there to make the fight more even. Adding in an economy cost will be a nerf to the bad guys. 
FYI, there’s a shield guardian amulet item in RotFM. It says making one takes 1 week and 1,000 gp in components, just as a point of reference. Also it has a 10 AC and 10 hp.

 This Guardian Amulet is a bit different than a Master's Amulet;

This particular shield guardian amulet is a rare magic item that requires attunement, and only humanoids can attune to it.

So the fight happened. Glad I didn't pull the "emergency reinforcement" the module had on standby in case the fight went too quick, two of the PCs almost died (I forgot the Rogue got hit with a double psychic damage Far Realm curse... that Phantasmal Killer got scary), and the NPC Psionic Ashenwight they found in there DID die. Maybe I was reading Ruxithid's stat block wrong, but it sounded like its Bonus Action to trigger the guardian to hit off-turn was already far an above plenty. Though I guess this implies that controlling the guardian's reaction to swing is limited to 1 hit instead of a full off-turn "turn".

Still not sure if it won't be too strong and overshadow the trio of PCs who are now level 7. I'll have to keep pondering on this. I suppose it could also just be yet another nail in the "why is this free pet better than the Beastmaster's?" case, but at least there's no Ranger in this party (though as a former Ranger main, this does annoy me a bit). 

I guess it also depends on if Chapter 6 and onwards of Phandelver and Below is balanced with the guardian joining the party. Gotta keep reading ahead anyway since that finally ended this chapter.

On the plus side, they seem to be incredibly excited that this thing kicking their ass is now under their control. They are already making plans to cast Fly on it to make it a gundam. Also, the Arcane Trickster is asking if he can cast Spider Climb on it and turn it into a Looney Tunes anvil... I guess that'd have to be a Dex Save on whatever poor soul is about to be pancaked... Maybe make it use the falling damage rules and make it a d10 on the target.

Also, wait, are you saying Ryme of the Frostmaiden also has one of those?
I was planning on running that after this module, but start Icewind Dale at level 12-13 and upscale every encounter.

Maybe two Steel Guardians won't be as broken once the party is above level 13.

I don't think that shield guardian has a control amulet, the one in the Ruxithid fight. It just says that the statue uses the stats of a shield guardian. So I don't think the players can take it as their own.

Yes, Ryme of the Frostmaiden does have a shield guardian that the PCs can get control of.