SebaClaude

SebaClaude is the slash ship between Sebastian Michaelis and Claude Faustus from the Black Butler fandom.

Canon
Both being butlers to vengeful children, Sebastian and Claude carry out their orders fluently and accordingly with rare chances of failure. In the uncanon second season of Black Butler, Alois has placed an order upon Claude to obtain Ciel. With this demand, the demon butler clashes with Sebastian whom's rightfulness is to protect the Phantomhive. With the nuisance of Alois's order aggravating the Phantomhive butler, the demon butlers host a bargain in which Ciel will achieve a second revenge on Alois and Sebastian will not devour only half of Ciel's soul. The bargain is then sealed with their blood absorbed by cut white roses.

Despite the bargain, Claude came in contact with Ciel's spilled blood conjured from Alois and Ciel's duel, becoming erratic and determined to steal the Phantomhive's soul upon tasting it despite his current contract with Alois. This forms a strong hatred in Sebastian towards the latter due to his claimed ownership of Ciel's soul.

Claude's unbearable desire for the soul drove him into stealing his own master's soul to mash Alois's memories with Ciel's. This causes Ciel to believe his former contract with Sebastian is now rightfully Claude's, abandoning the Phantomhive estate and settling with the Trancys'. With the rare chance of consuming the perfect soul slipped from Sebastian's fingers, Sebastian formed a deep resentment towards the Trancy butler, driving him into repossessing his master with anger. Much like Alois and Ciel, the butlers declared a duel in the "Demon's Sanctuary" with the only weapon being the demonic sword constructed by Hannah's body. The loser in the battle shall lose his life. Unfortunately, the duel has ended with Claude's failure, the demonic sword ramming through his chest with Sebastian on the other end.

Before these events, it is confirmed that the butlers acknowledged each other in the past, though their relationship status then is unknown.

Fanon
Much like Cielois, SebaClaude isn't considered or acknowledged as much as most SebaClaude shippers want it to. This is highly due to the demon butlers' intense rivalry and Claude's unforgivable actions of cheating to steal what was rightfully Sebastian's. Shippers find this pairing entertaining due to the shared demon ethnicity and very similar personality aspects and behaviors. Fans of the Enemies to Lovers trope show desire for this pairing as well.

Due to Claude's loss in the duel stealing him of his life, SebaClaude fanfics created AUs of his survival with Sebastian reluctantly refusing to pierce him with Hannah's demon sword.

Trivia

 * Sebastian and Claude are the only male demons to have occupations as butlers.
 * Claude has very similar physical complexions to William T. Spears, a former rival of Sebastian.