Define imperious as in 'an imperious necessity drove him forward on his quest'
Define despondency as in 'he fell into a state of deep, uninterrupted despondency'
Define arduous as in 'the long and arduous journey through the frozen mountains'
Define ephemeral as in 'human history appeared as a brief and ephemeral flash in cosmic time'
Define austere as in 'they lived under the austere discipline of their new order'
Define venerable as in 'the venerable monuments of a long-forgotten civilization'
Define tumultuous as in 'the species endured a tumultuous period of war and renewal'
Define imperturbable as in 'he maintained an imperturbable calm despite the surrounding chaos'
Define singular as in 'he was a man of a singular and unpredictable temperament'
Define enigmatic as in 'he left behind an enigmatic smile and no further explanation'