I am a poet. I understand that the rules of proper English grammar are not always followed in poetry-writing. However, I don't want to be "over-the-top" wrong. I have a line in one of my poems that reads "and land directly at the feet of He who sits on high". Is this correct? Or should it be "land directly at the feet of Him that sits on high".
The trick is to find out what the true object is. For example,Originally Posted by BRICEERIC
 I gave the book to him.
gave is the verb and it takes two objects: (i) the book functions as the direct object and (ii) to him functions as the indirect object. The preposition to also takes an object: him. Now, if we replace him with a noun phrase, say, the person who wanted it, roles change:
 I gave the book to the person who wanted it.
=> the object of the preposition to is the person who wanted it.
=> who is the subject of wanted.
=> who refers back to the person.
ungrammatical: I gave the book to the person *whom wanted it. (whom functions as an object, never as a subject.)
 . . . the feet of he who sits on high.
=> who is the subject of 'sits'
=> who refers back to he.
ungrammatical: the feet of *him whom sits on high. (whom refers back to him, but whom cannot function as a subject, as the subject of 'sits'.)
Hope that helps.