Context: I have recently started a campaign with a new player that comes from an AD&D background. He wishes to convert and import his character to the campaign. We did this once, and I helped him with the conversion and checked his stuff; he got 2 magical items at 3rd level. A bit much, but I didn't mind as they were not game-breaking (ring of immunity to poison and mobility boots[longer jumps and a bit of speed up]). Then this character died, and he asked to import and convert another character. This time, I asked him for his stuff and he says that he has a magical sword and a giant strength belt as a 6th level character. This is, IMO, very strong, but again not game-breaking, or so I thought; I tweaked a bit of the encounter and it went fine.
The thing is that I gave him a full plate armour and then he says that his armour class was 23. I was surprised because Max AC without magic is 20 - 21 (with defence combatDefense fighting style, which he has).
The Item: a magical sword with +2 attack bonus, magical damage, +2 fire damage, and +2 AC.
In my understanding, at this level, having an AC of 23 (25 with shield of faith, he plays a paladin) is game-breaking. Most monsters encountered at this level get attack bonus of +4 to +6, meaning that he will only get hit on crit by most monsters, or that I have to send exceptionally powerful monsters for this level, which may not be appropriate for other players.
Is this AC game-breaking at level 6?
I started to talk about this. and it went bad. He says that it is only 8 % increase of AC and did do much and that he gains his sword on a very deadly quest etc. Maybe it is a good computation in AD&D setting but in 5e I feel it is not.
For now, I have come up with the following arguments why the sword providing +2 AC is overpowered:
- AC doesn't scale the same, and 21 is already the maximum you should expect to be. A venerable red dragon AC is 22.
- AC increase is extremely costly, with non-magical means; there are only two ways: combat style and feats (heavier armour type or the Medium Armor Master feats (+3 dex))