It's the letter D of course! Haven't you heard of Jason D?!
what is the next letter e h l q
The main character in the next Percy Jackson and the Olympians series is named Jason. There is not much information given on him, or the next Camp Half blood series.
X
S
c
Q
c is next
y
E
S
T
The next (and last) letter is C. Just as a countdown counts backwards, this series is "countdown" spelled backwards.