Constructs are great. Their main downside for being a BBEG is independent agency as most are created to be automatons. But that could be exploited. you could have a Marut whose golden tablet was corrupted or was written so badly that it has misinterpreted it's goal and is now on the warpath against entire cities.
You could also have a construct that has an intelligence in it. Some constructs are simply imbued with sentience, like Warforged. And magical AI works but you could have a caster that made a modified Magic Jar and created a construct to house their soul. Now that wizard has a new body to conquer the world and you can bite his shiny metal ass!
But the immunities are nice. Generally the physicality is fixed so you can't change their form. Exhaustion, poison, paralyzed and petrified generally don't work either.
Lorewise maruts are damn near incorruptable, but even just a rogue modron could be cool if done right. A marut is also unlikely to misinterpret the contract, but the signatories might have.
“[…] A marut enforces what is written, not what was meant by or supposed to be understood from the writing. The Kolyarut rejects contracts that contain vague, contradictory, or unenforceable terms. Beyond that, it doesn’t care whether both parties understand what they’re agreeing to.”
There’s also a reason they’re called “nigh-unstoppable inevitables”.
I guess now that I think about it, a simulacrum is a construct as well, but I’d have a hard time counting one of them. Most powerful constructs tend to be golems or a colossus like the Walking Statues of Waterdeep, not independent thinkers, just powerful brute force minions.
Spoilers for a Discworld book: >!The "antagonist" in Feet of Clay was a Golem that the creators made with unclear instructions that essentially sent it insane!<
The characters in the top panel are from a New Yorker cartoon. I think it’s supposed to be blushing?
I may have posted the original meme in a discord server somewhere, but I put it on imgur now so it’s available publicly. YoumaybeabletoseewhytheideaofaBBEGbardbroughtthistomind.
Edit: Found where I originally put it on imgur. Huzzah, the history is preserved.
I was disappointed that in Portal 2, GLAdOS was revealed to be spoiler an uploaded human mind with a tragic backstory. IMO that took away a lot of her agency.
I’ve been considering doing something with a rogue decaton. Maybe drop some rumors or hints that make it sound like a beholder amassing a construct army, only to reveal it’s a decaton with a bunch of pentadrone minons that never question why their boss has started drifting into LE territory with the orders lately.