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Does anyone remember a giant metal type monster from one of the older versions of D&D? I have a vague memory of some sort of monster that the gods of death might send in to balance out the universe if someone (or someones) had cheated death enough times that they were unbalancing the universe.
I was trying to track it down recently and can't find it, so I'm wondering if this is a real thing or someone's homebrew I'm misremembering.
There is not a canonical "D&D" answer. The answer differs per campaign world.
I know it's a little weird - the D&D 3 core books don't present themselves as a generic system per se; they hint at a shared cosmology with the gods, certain roles for the races, etc. that makes it seem like there's a larger world there. But it's just a hollow shell, to be filled in either by yourself or by the specific campaign worlds (Forgotten Realms, Greyhawk, Golarion, etc.).
Now, in 3e/3.5e Greyhawk is the default setting (they didn't follow through on that real well but it was stated) so you could look to the Greyhawk answers as the canonical defaults. I don't know that they ever answered the "but where did they all come from" questions, and I was a Greyhawk aficionado for decades. Canonfire! would be the place to research that. If you're looking for Corellon specifically, try this wiki article. Most campaign worlds are deliberately cagy about how exactly the beginning of all things transpired, however, and Greyhawk is especially so.
Best Answer
You are most likely thinking of the Marut, a powerful construct/outsider (depends on the edition) that hunts those who cheat death, and has been in every edition since AD&D.
What they are like for each edition:
1e AD&D - In 1e, They are the most powerful variety of inevitables, and although only a little information is given, it is mentioned how 'severity' plays a large role in their decision to pursue a target (sacrificing a city for immortality will likely draw their attention, while casting Ressurection once will not). They have a minimum of 15 HD, and advance up to 45. Although created with a divine mandate, these Maruts are constructs.
2e - 2e Maruts are very different from their original representation; they are celestial emmisaries who serve no particular purpouse, although their powerful single-target damage leads them to be sent to hunt individuals. They don't even have their signiature lightning and thunder fists in this edition! They do still have 15 hit dice, though.
3.5e - 3.5 Maruts are carbon copies of their 1e counterparts.
4e - Maruts are more similar here to their version from 2e, semi-celestial creatures that may be on any of a variety of tasks, but in the end serve the order of the universe as a whole.
5e - 5e statblocks are less readily available (it is the current edition), but this version seems more similar to the 2nd and 4th edition versions than the 1st and 3rd, being an enforcer of contracts rather than of death.
Pathfinder - Pathfinder seems to take off of the 3.5 interpretation, which isn't surprising. This holds true for Pathfinder 2e as well.