The SeaDex is a Google Sheets document that lists the best releases (audio, video, and subtitles) for certain anime.
SeaDex has allegedly been said to have groomed notorious muxer Yuri, bringing about the zero QC revolution on Nyaa.
The site has also, in the words of some, been said to frequently pick the worst subtitles available
Due to the fact that many Blu-rays and DVDs of anime look bad, there exist individuals, known as encoders, who take these official sources and use tools such as Vapoursynth (and previously AviSynth) to remove video artifacts. They then release these new versions of the anime, which look better than the official releases, onto sites like Nyaa.
However there also exist another group of encoders, known as mini-encoders (as of late most notably those favoring AV1) who take the same official sources and make the anime look worse, often introducing many compression artifacts in the pursuit of lower file sizes. However sometimes this is also caused by a lack of competency.
In order to help accurately differentiate between these various releases and help people pick out good ones, SeaDex was born. It was first founded by a user called SeaSmoke, who later transferred ownership over to the user V01.