Choice Chamber is a real-time procedurally generated game developed by Studio Bean that is "crowdsourced", i.e. the progression in the game is tied to people writing keywords in the connected Twitch chat. The game was released on 16 Jul 2015.
The sigil was added in an update 15 Sep 2016.
In a normal playthrough you can only get a glitching heart after the cheating heart, which requires a shop that doesn't occur earlier than level 10. But using the debug mode to travel through rooms you can get the glitching heart on level 8. You'll see a pulsing sigil over the chest if you get to it.
Alternatively, if you manage to hold 2 cheating hearts in room 80 and use one to go back to room 8 and do it again, it can theoretically let to the sigil without the debug mode ever invoked.
As you obtain the glitch heart from the chest in room 8 a message that looks like an error pops up.
That message contains a bunch of random characters, but the string "say nothing" is always present, as well as the strings "ERROR" and "room8."
Saying "nothing" in a linked twitch chat while in room 8 causes the chat bot to emit a reversed imgur url.
The image, also mirrored, is cryptically asking you to go back 501 rooms from the beginning and into room -500.
As there is no door in the trophy room, you can go into negative ones only with debug. It's also impossible to travel through them without debug, since their doors close on both sides.
Once in the room -500 (which features a big open space unlike the previous 499 rooms) you need to wait about 30-60 minutes while the map piece assembles itself (due to how the algorithm is written, it slows down in the end, as it is pretty much complete).
The sigil is stored in the image BitsLossless2 (801) of the game's SWF.