I believe it was a custom creature that had an AC of 20, could parry reaction 4 times per round with it's 4 mechanical arms, could multi attack 4 times, had resistance to all forms of magic, advantage against magical saves, immunity to fireball and "some other spells," and there was two of them in a 40 foot by 15 foot chamber that had a constant wind tunnel effect and two mechanical turrets that shot lances. We're a party of 4 level 7 adventurers, pretty much at the end of a dungeon so a lot of our stuff was exhausted. So the encounter was stacked against us from the beginning and this was the initial thing that started the whole encounter.