Soriel

Soriel is the relationship between Toriel and Sans in the Undertale fandom.

Canon
At one point, Sans describes how he used to speak with a woman who was on the other side of a wall and they enjoyed puns together but one day she just disappeared. While it isn't directly stated, it's heavily implied that this woman is Toriel. It is also proven in the true Pacifist ending that she is indeed the woman he has been talking about, as they recognize each other's voice.

In the Pacifist Route, Sans and Toriel meet each other and discover their common like for puns.

In the Exiled Queen ending, Toriel starts to live together with Sans (and Papyrus if he is alive).

Fanon
People like to pair Toriel and Sans due to the fact that both of them like puns. Towards the end of the game, they enjoy telling puns to each other, annoying the rest of the characters.

Its biggest rival ship is Asgoriel, the ship between Asgore and Toriel. Soriel shippers usually point out that Toriel refuses to get together with Asgore at the end of Undertale.

Fandom
AO3

DeviantArt
 * soriel tag on DeviantArt
 * sansxtoriel tag on DeviantArt
 * soriel_undertale tag on DeviantArt
 * Sorihell fanclub on DeviantArt
 * Toriel-and-Sans fanclub on DeviantArt