The Collaborative International Dictionary of English v.0.48:
Thyself \Thy*self"\, pron.
An emphasized form of the personal pronoun of the second
person; -- used as a subject commonly with thou; as, thou
thyself shalt go; that is, thou shalt go, and no other. It is
sometimes used, especially in the predicate, without thou,
and in the nominative as well as in the objective case.
Thyself shalt see the act. --Shak.
Ere I do thee, thou to thyself wast cruel. --Milton.