Omission of the relative pronoun

The relative pronoun is usually omitted when it is in the accusative case.

Few and short were the prayers (that) we said.
I am the monarch of all (that) I survey.

The tendency to omit the accusative relative is common in spoken English. In written English it is considered inappropriate.

The omission of the relative pronoun in the nominative case is unusual except in colloquial English.

There is somebody on the phone who wants to talk with you. (NOT There is somebody on the phone wants to talk with you.)

Omission of the antecedent

In older English the antecedent of a relative pronoun was sometimes left out.

Who laughs last laugh best. (= He who laughs last laughs best.)
Whom the Gods love die young. (= Those whom the Gods love die young.)
Who works not shall not eat. (= He who works not shall not eat.)

Position of the relative pronoun
To avoid confusion, the relative pronoun and its clause should be placed as near as possible to its antecedent (the noun or pronoun to which it refers back).
Compare:

The boy who won the scholarship is the son of my brother Charles. (The boy won the scholarship.)
The boy is the son of my brother Charles who won the scholarship. (Charles won the scholarship.)

Compound relative pronouns
Pronouns which are formed by adding ever, so, or soever to who, which and what are called compound relative pronouns. Examples are: whoever, whichever, whatever, whosoever, whatsoever etc.
Note that compound relative pronouns usually have no expressed antecedent.

Whoever diggeth a pit shall fall therein.
Whoever comes is welcome. (= Any person who comes is welcome.)
Take whichever you like.
Whatever he does, he does well.