Effects of giving a player a Shield Guardian

[Cheesegear]

Like what it says.

How does adding a Shield Guardian to the party - or rather, a specific player - affect the outcomes of combats?
Not sure how Deadly I need to go.

[heavyfuel]

What is the party level?

A shield guardian would be amazing in Tier 2/early Tier 3. By late Tier 3/Tier 4 it's no big deal.

[MaxWilson]

Like what it says.

How does adding a Shield Guardian to the party - or rather, a specific player - affect the outcomes of combats?
Not sure how Deadly I need to go.

What combat outcomes are you aiming for? Are you trying to ensure that players have a 50% chance at TPK if they don't fight smart (call that True Challenge), or that they end the adventure with less than 50% HP and only a handful of spell slots left (call that Out Of Gas), or are you just trying to make it hard enough to be entertaining?

If you're just trying to provide some entertaining Combat As Sports play for the players, I'd probably just build the fights as if there were 1 extra PC and call it good. Obviously Shield Guardians are extremely good in Out Of Gas scenario due to regeneration, but I doubt you're actually trying for that.

[Lord Vukodlak]

It’s certainly a big advantage. In the prebox campaign
Princes of the Apocalypse an NPC had a shield guardian which meant soon the PCs had one. Because why wouldn’t we spare no effort in preserving such a prize. And it was a big help in further fights.

If you’re going to let them have one make them work for it. First have the amulet require in a attainment slot. Second they should have to fight some guy to get the amulet. Which means fighting the shield guardian without killing it.
And even after getting the amulet the shield guardian should continue trying to carry out its last command. Likely kill these guys. Until a pc attunes to the amulet and become the new master.

[da newt]

If the shield guardian was a PC what level would it be?

If you can answer that question, you can answer the OP question.

As a creature w/ 15 hit die, and a CR of 7, I'd SWAG it's equivalent to a single PC of 12-15 lvl. Adjust accordingly.