Forewarned

From The Booke of Meery Riddles, 1629:

A soldier that to Black-heath-field went,
Prayed an astronomer of his judgment,
Which wrote these words to him plainly,–
Thou shalt goe thither well and safely
And from thence come home alive againe
Never at that field shalt thou be slaine.
The soldier was slaine there at that field,
And yet the astronomer his promise held.

How?

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