Learn English – What comes in between predecessor and successor

We don't often need it, but the word is nonuplet.

Next after that comes decuplets, who according to the Guinness Book of World Records were born at Bacacay, Brazil on April 22, 1946. (I must admit I'm somewhat sceptical).

It's hard to be dogmatic about the spelling of a word that probably doesn't make it into many (if any) 'authoritative' dictionaries, but I will just say that - Wikipedia notwithstanding - I don't have much time for decaplets. Here's an NGram showing that not many others do either (in fact, there are 5 instances of decaplet in the NGram corpus, as opposed to 2450 decuplets).

It comes from Latin successor, which means roughly the same. The verb comes from succed-o, which also means roughly the same.

The root of the Latin verbal stem ced- ("move away, cede") is the Proto-Indo-European root *ke-, "here, this" (deictic/pronominal stem), plus *s(e)d-, "sit, set, settle" (compound *k̂e-zd-ō)—so says the Oxford Latin Dictionary (1982): Pokorny and Ernout (1959) are unclear.

The prefix sub-, "from under", is added to form the meaning "to go sit/move from under" for succedo, if you will forgive me this awkward translation. It is a phonological rule in Latin that -bc- is normally assimilated into -cc-.

The supine stem of a Latin verb—the stem used to make past participles—is normally formed by adding -t- to the verbal stem: *cedt-. Phonology demands that -dt- (and -tt-) at the end of supine stems normally change into -ss-, hence the supine stem success-. The past participle is successus, roughly "succeeded".

The suffix -or is used in Latin to make an agent out of a verbal stem, just as in English -or and -er: hence Latin successor, "successor".

In English, we sometimes borrow words from Latin directly, or from Latin through French, like successor, while at other times we only borrow the verb and make our own agent word, like revoker (instead of revocator). But we usually use the Latin noun if you also use the Latin verb: proceed, processor; permit, permission, etc. etc.; sometimes we use both, as in merge, merger, immersion.