The 'grandeur' here refers to the beauty of the noble deeds.The "mighty dead" are the warriors who have sacrificed their lives for some noble cause. They have left behind the beautiful legacy of their brave acts. Their selfless sacrifice for their nation and deeds for the welfare of mankind still continue to inspire us.
Therefore, on every morrow, are we wreathing
A flowery band to bind us to the earth,
Spite of despondence, of the inhuman dearth
Of noble natures, of the gloomy days,
Of all the unhealthy and o’er-darkened ways
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