Possible ARG discovered involving multiple Steam games

There appears to be an ARG connecting several ~~Steamgames~~ game-related things together with one symbol originally found in a file called eye.jpg with this image. So far this symbol seems to be accompanied by a puzzle to get a new image. To make progress in this ARG we're going to a lot of people on the lookout for this symbol as you play various games so we can solve the puzzles surrounding them.

Alert: For up-to-date information on this ARG please visit the Eye-Sigil ARG page on the Game Detectives Wiki, and to discuss the ARG you can join the GameDetectives Discord server. Videos for solutions can be found on Eye Sigil ARG's Youtube channel.

No.	Name	Description
1	Crypt of the NecroDancer	Around May 29th 2015 a file called eye.jpg was added to Crypt of the NecroDancer directory. Using a hex-editing application Puzzle Piece #1 was found inside of eye.jpg. This image seems to be a circular puzzle piece.
2	Mini Metro	Around February 13th 2015 a secret map was added, and could be accessed by following these instructions to find and click a "eye". An email sent to the developer confirms that there is a puzzle, a solution, and that it is solvable. The secret map has stations shaped like the "eye", and the puzzle seems to revolve around recreating the "eye"'s shape by drawing lines. My progress for the puzzle is here, but was solved to reveal Puzzle Piece #2. /u/Pentamerous has provided a video tutorial to reveal the piece.
3	Legend of Dungeon	Clicking on a dwarf in the Options menu pops up a version of the eye-sigil. Datamining lead to Puzzle Piece #3. Portalcrafter found the puzzle piece in-game.
4	The Magic Circle	I'm unsure when this "eye" was added. /u/Tambox discovered a "eye" in The Magic Circle and provided this image and video. In July 15th Puzzle Piece #4 was found and discussed here, but was only connected to this ARG now.
5	Neon Struct	Eye-sigil was found on Stranger's shirt in-game. Puzzle Piece #5 was first revealed through data-mining, but Crauss obtained the piece in-game later. To get the piece in-game, play through the game talking to all strangers and getting all geocaches saving one displacement orb for the end, and at the end throw the displacement orb past the ending trigger. An image of the piece can be seen here. (The numbers are double hex, and says "NEON STRUCT is a video game.")
6	Mos Speedrun 2	Sigil found on a wall here. Puzzle piece not found completely, but in separate parts during a part with "glitchy" tiles. JSHC using Tom's savefile found Puzzle Piece #6 by falling down between the 2 gray blocks in level 3-3 and going down and left.
7	Flickers	A free non-Steam game that can be downloaded here. Sigil can be seen on a tree in the background here. Puzzle Piece #7 was initially found by crauss and anonon by modding the map file. (Isolated) Legitimate solution is to speedrun from the checkpoint perfectly using the strategy in this image and this video.
8	Sokobond	/u/AmitaiGaunt (Amitai on Discord) found the sigil AND Puzzle Piece #8 about half a year ago, but only found out about the ARG now. Whenever you solve a level with a solution involving hydrogen molecules the sigil appears with a count of such solutions you have made. Upon achieving 12 such solutions the puzzle piece appears. You can read a longer write-up from Amitai here.
9	Moon Hunters	/u/theDrunkenManatee (theDrunkenManatee on Discord) found the sigil during a Twitch stream. In multiplayer, two people have to perform a specific pose while standing on the sigil to obtain Puzzle Piece #9.
10	Soda Drinker Pro	/u/OuroborosJones found the sigil in the corner of the 2nd level. To find the puzzle piece: "After you poke through the wall in the corner, you can sort of skirt across to the left and fall on the walkway. There's a light on one end you walk into that causes the main level to move away and reveals a massive pulsing sigil, and then when you reach the end of the walkway and turn back you see the puzzle piece." courtesy of @Immortius. Video
11	Bellular Hexatosis	Puzzle Piece #11 found by clicking the colored mushrooms found elsewhere in the game in the order the sigil displays after spinning.
12	Slide the Shakes	/u/TobiasPak found the sigil on Level 88 in Slide the Shakes. Morse code on each stage of the level spells out "ARCHUSB". USB found at an arch from the developers previous ARG resulted in a USB with Puzzle Piece #12 on it.
13	Souls of Darkness	@eltrov found the sigil in the book Souls of Darkness. Sigil can be seen on the website or in the book. Viewing the image of the map on the website gave a code of numbers in Webdings. The numbers were a book cipher to be used on the GAME SOLVERZ HINTS after each of the 11 chapters in the book. It led to 'osdPcDotpng' which led to the image link of the Puzzle Piece #13.
14	Kingdom of Loathing	A non-Steam webgame. The eye-sigil was found looking like this, and dates back to October 2014. After 6 months Puzzle Piece #14 was found by taking a list of item drops in the Laboratory except for Monster Manuel items, sorting the list alphabetically, taking the last letter of each entry in the list to get 'nlry9htdotgif' and then using that in the URL.
15	Quadrilateral Cowboy	Sigil can be seen above 9 clocks on the Malta Stock Exchange level. One of the monitors on the "Malta Stock Exchange" level says "MISSING CHUNKS". Within the games files, the code that renders text on this screen needed to be modified: "DELETE_THIS_CHUNK" part should be deleted and the missing chunks that are spread in comments of other scripts should be "restored". Then Puzzle Piece #15 is displayed on the monitor, drawn by rectangles defined by the chunks.
16	There Came an Echo	Sigil is visible on the wall in level 5, near a point called Foxtrot 6. Datamining the sigil revealed an equation (solution=EIEIO), leading to "Old McDonald Had a Farm" phrase. Using this voice command in-game in front of where the sigil is shows Puzzle Piece #16 on screen.
17	Duskers	Sigil can be seen by hitting F1, F2, F12 or Del on launch, then hit Enter on seeing %)!@(#$@#$(&& and it appears in the top left for a split second. Choosing the %)!@(#$@#$(&& option opens the browser with an URL containing string of characters 0-)$(_]]@!!+@=. This string is mentioned in one of the in-game logs that also hints to another similar string, leading to the second URL with a login form. Currently the legitimate credentials to this form are not known, but Puzzle Piece #17 was found in that folder under the filename piece.jpg.
18	Clockwork Empires	Sigil can be seen in the promotional drawing on their website update page. Same art appears in-game at certain "strange artifact" events. Puzzle Piece #18 was hidden with OpenStego steganography software with the randomLSB algorithm and the password 191761112 in the image with the sigil. The password is composed of the amounts of times the symbols in the promo image appear on the Phaistos Disc in order (19, 17, 6, 11 and 12).
19	You Have to Win the Game	Sigil can be seen on the Listen to the Wind screen of the Extra Spicy mode near the end of the campaign. The room with the sigil has a changed title — a clue to a different room, which will also have a sigil upon visit. Following the trail of clues in changed titles, player arrives to the winning screen with 7th sigil and title Backmask the Myth, what refers to the screen Hydra is Myth of the original campaign. By playing the original campaign and entering the ARDYH password near the end, the player is teleported to the series of secret rooms marked by the sigil. One of the rooms there (Sadness) has Puzzle Piece #19 on the pause menu.