While Warriors and Beastmasters may obtain skill level 240 earlier than level 71, Decimation must be quested once base skill level 240 (without merits) and job level 71 is reached (requires the ability to wield Pick of Trials).
Axe skill level 240 is obtainable by the following jobs at these corresponding levels:
Decimation comes from the Latin prefix 'dec', meaning ten. In Ancient Rome, decimation was an extreme form of military punishment, wherein a group (usually a 'cohort'- a force of 600 legionaires) would be divided into subgroups of ten. Each group would draw lots. Whoever took the lot would be executed by the other nine. Because the groupings and lots ignored rank and social status, everyone was subject to the risk of execution; this, along with the fact that you were being killed by your peers made decimation extremely daunting and personal. Cultures extended its use to rebellious provinces and subjugated people, taking entire populations into subgroups and executing every tenth person to establish dominance, control, and overall tyrrany.