In archaic|lang=en terms the difference between hereunto and thereunto
is that hereunto is (archaic) unto this; up to this time; hereto while thereunto is (archaic) thereto.
As adverbs the difference between hereunto and thereunto
is that hereunto is (archaic) unto this; up to this time; hereto while thereunto is (archaic) thereto.
hereunto
English
Adverb
(-)
(archaic) Unto this; up to this time; hereto.
thereunto
English
Adverb
(-)
(archaic) thereto