For people into kanji, this is kanarazu (必ず、かならず) . The kanji is used for many things requiring “certainty”; of these hitsuyou (必要、ひつよう) for “absolutely required” (or must-have, as the case may be) is probably the one most people have seen in romaji.
This is a so-called “kun reading,” that is, a native Japanese word associated with a kanji (or Chinese character).
The compound verb is modotte kuru, from modoru (戻る、もどる) for “to return” and kuru (来る、くる) for “to come.” Fortunately, this is very easy to explain: this means to come back, in English phrasal verb terms.
So literally, “I will come back.”
Figuratively, “I’ll be back.”
This may be a short phrase, but it will strongly challenge early learners due to the presence of a compound verb and “kanarazu” not being one of the earliest things taught. – J