The most common and most likely answer to this crossword clue is the 8 letter word DOOMSTER
I've seen this crossword clue in The Daily Telegraph
D
O
O
M
S
T
E
R
Buddy explains!
The straight part
We know that "who tells us the end is nigh" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case who tells us the end is nigh = doomster.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does DOOMSTER mean?
DOOMSTER noun- Someone who predicts doom
- A judge; a deemster.