Its a very good riddle.
I have been stuck on it for like 2 hours now and can't figure it out. This is the riddle- Though any number of words can complete the series, there is one which would no longer complete but also explain the whole series. ONSET TWIGS ___ FOSTER FISHES SINISTER SEVERE

Till now my progress is that it is a 5 letter word(was given as a hint) , and it starts with TH, as the sequence is like ONe, TWo, THree, FOur, FIve, SIx,SEven.
Please help.

Question

Its a very good riddle.
I have been stuck on it for like 2 hours now and can't figure it out. This is the riddle- Though any number of words can complete the series, there is one which would no longer complete but also explain the whole series. ONSET TWIGS ___ FOSTER FISHES SINISTER SEVERE 

Till now my progress is that it is a 5 letter word(was given as a hint) , and it starts with TH, as the sequence is like ONe, TWo, THree, FOur, FIve, SIx,SEven.
Please help.