Sonadow

Sonadow is the slash ship between Sonic and Shadow from the Sonic the Hedgehog fandom.

Canon
In most canons, both Sonic and Shadow share an arch-rivalry, starting as enemies when they first met up.

Games
When they first met as enemies in Sonic Adventure 2, Shadow believed Sonic to have been copying him, with a mutual belief from Sonic, but later they became bitter arch-rivals after Shadow's amnesia. Since meeting each other, there has been a never-ending competition between the two, and their battles can range from friendly races to fierce death battles. In any case, one always wants to outdo the other.

Now there's a mutual respect between them despite their ferocious arch-rivalry and frequent fights. Overtime, Shadow has become more confident about Sonic doing what is right, and Sonic has become more acceptant of Shadow's violent attitude as well.

Archie Comics
Their relationship is similar to the games. However, there was once a moment when Sonic felt sadness for his arch-rival as Shadow had presumably perished after the destruction of Letter of Gabriel, but after the timeline was reset, Sonic immediately called and checked up on Shadow, and was elated to find him alive.

Sonic X
Their relationship is similar to the games, but at the first time they met Shadow showed interest in meeting Sonic, noting their similarities and mocked him as a weak opponent as he was able to defeat Sonic in combat. However, he later changed this view about Sonic after seeing Sonic using a Chaos Control. Shadow got impressed by Sonic and had come to respect him as a rival, ever since.

Sonic Boom
Like other canons, both are tense arch-rivals but directly they never showed partnership once. However, Shadow has a small degree of respect to Sonic, and in fact, fighting against him is one of the few things Shadow ever shows an interest in.

Fanon
While Sonadow isn't the most popular ship for either of the characters, it's the most popular slash ship in the Sonic fandom and it still has a large fan-following.

Fandom
AO3

DEVIANTART
 * sonadow tag on DeviantArt
 * shadonic tag on DeviantArt
 * sonshad tag on DeviantArt
 * sonshadow tag on DeviantArt
 * shadsonic tag on DeviantArt
 * sonicxshadow tag on DeviantArt
 * shadowxsonic tag on DeviantArt
 * SonicXShadowClub fanclub on DeviantArt

TUMBLR
 * sonadow tag on Tumblr

Trivia

 * Referring to the ship as Shadic is incorrect, as this is the name used for the hypothetical fusion of Sonic and Shadow.