Question

I know this is not a mathematics question but it does involve use of ONE,TWO, THREE etc. I'll explain how below. 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.

Answer 1

THRIVE?

Answer 2

http://www.scrabblefinder.com/starts-with/th/#words5

Answer 3

THROW

Answer 4

thrive is a good guess i like throw better

Answer 5

throw is 5 letters

Answer 6

yea ik lol

Answer 7

i dont get the riddle though its kinda random

Answer 8

lol ikr