Don`t have to use Variables.
You could use something like this in Q-Function.
[@ONKILLMOB(OmaWarrior)]
#IF
CHECK [120] 1
RANDOMEX 300 = 5
#ACT
MESSAGEBOX "You have now proven to me your ability, return to me."
SET [121] 1
SET [120] 0
BREAK
#ELSEACT
BREAK
You would first need an NPC to create a flag to the character for the quest to start.
#ACT
SET [120] 1
#SAY
Go and prove yourself to me. \
<I will/@exit>\