CHESS IS MADE UP. Rules are made up. You can do whatever you want.
Ahem; It's a bit strange, I think, that there seems to be no commonly accepted fairy chess piece like the Ruin insofar as it combines its two main qualities of movement and capture. I don't think I'm a genius for coming up with it. I'm sure I'm not even the first to do it, it just hasn't stuck.
The Ruin moves to any open square, and cannot capture or be captured. That's it. In veney, there is more to be said about it, but that's because veney is overcomplicated. In a game of chess, those are the only qualities of the piece.
If this all looks like crazy talk to you, the reason I am "surprised" to find some inane little chess thingummy didn't already existing in the world just for me is simply that there very much is a long history of nerds exactly like me doing and cataloguing this stuff, and an even longer history, like potentially thousands of years, of wildly various chess pieces doing their weird little moves.
I have done a bit of searching, though admittedly not super extensively. My sources are basically the Wikipedia page for their List of fairy chess pieces, some poking around in a few of the online sources they cite, and finally the monumental Classified Encyclopedia of Chess Variants, [PDF] by D. B. Pritchard. (It's "classified" for some reason because it's the second edition of Pritchard's Encyclopedia of Chess Variants, completed and edited after his death by John Beasley.) This book is a little difficult to search for a hypothetical piece, because it is mostly (naturally) organized by game, categorizing them based on key qualities. There are a few relevant categorizations relating to movement and capture; checking through those games didn't turn up anything. It seems like the key piece of vocabulary used for the relevant capturing definition is "inert;" a search turns up many results for pieces that are inert in certain conditions, but none permanently. Searching for the ability to move to any square is less easy. It is possible that a ruin-like piece could be found somewhere in one of the many obscure games described therein, but even if so, I still find it weird that a piece like this hasn't made it into the fundamentals of chess variants.
Here's what I've been able to find in the way of preexisting fairy pieces that approach that kind of inertness, all from the above linked Wikipedia list.
Diplomat - "Does not capture, cannot be captured, cannot move, but it saves from capture any adjacent piece to it." So, inert, yes, but can't move at all. While it does block off a square, the point of the piece seems to be primarily the use of its special power.
Dummy - (historically problematic name acknowledged) "A piece with no moves at all. (...) It can be captured." Half-inert, only provides blocking until captured, doesn't move.
Pyramid - "Never moves, cannot be taken. It blocks its square." - Inert but never moves, may not even belong to either player; really highlights the theme here of pieces invented for use in chess problems. [EDIT: See also the "Wall" and "Transparent Wall" which are less game pieces than they are parts of the board]
Zero - "A (0,0) leaper. Jumps and lands on the square where it stands. It allows to pass a turn. It can be captured. Invented by A.S.M. Dickins." This is neat widget that I love for reasons unrelated to the subject at hand. I have to include it, but unfortunately it doesn't "move" per se and is only half-inert.
These are all great, but are any of them strategically manipulable inertness? No. The ruin feels to me like a fundamental property totally different than anything they're going for in all of the above. There is however...
the Go-between (行人) - (from the Game of the Seven Kingdoms, a seven-player xiangqi variant) - "Moves like a Queen, but cannot be captured nor capture. Also known as a Jester" (the latter in a "Faerie Chess" game for sale by publisher Brybelly). This is pretty much it! It even moves on the somewhat higher-end of mobility, though when we're talking fairy pieces there is always queen+knight, leaping queen, etc.
Seeing the last one, it occurs to me that being able to move to any open square might easily break the game unless the piece is inert, and inertness is not very interesting or playable unless it has some other quality, like free mobility. So, that's why these two qualities go together so well in my head.
The Duck
AHHH I FORGOT ABOUT DUCK CHESS. I forgot about the goddamn duck. Okay, that's it, that's the thing already. Invented by someone named Dr. Tim Paulden. It's a duck. It's fine.
But! the duck is a shared piece, which adds a whole other dimension and brings us, all the more, right back to the original question of a missing fairy piece. I feel like it being a yellow duck and self-consciously an oddball is sort of pointing to a strangeness that would be even moreso exposed by giving each player their own black duck or white duck. At that point, why would they be ducks, you know? I dunno. It's interesting to note that the Duck is not included currently in the Wikipedia list, despite its popularity. I am inclined to surmise that this is because it's not thought of as a fairy chess piece; it's a computer duck. It's a variant game with a duck in it. The duck is cute and I'm not mad, I'm just saying there's still something going on here.