suru verb (special), intransitive verb
be inevitable; be certain
Archaic intransitive use meaning something is bound to happen or cannot be avoided. Not used in modern Japanese.
古い文献では「必する」が「必ず起こる」の意味で使われることがある。
In old texts, 必する is sometimes used to mean 'to be inevitable'.
suru verb (special), transitive verb
resolve to do
Archaic transitive use meaning to make up one's mind firmly. Obsolete; modern equivalents include 決心する or 覚悟する.
「必する」は古語で、現代では「必ず～しようと決心する」とは言わない。
必する is an archaic word; in modern Japanese one would not say 'resolve to definitely do' this way.