I come with a seal of approval

Question

I come before the end,
I am not your friend.

Brothers, have I three,
They shall make you flee.

The first is white,
And ready to fight.
Posies in his hand,
All are sick upon the land.

The second is red,
Peace has fled.
You call him to your aid,
But his price must be paid.

The third is black,
You can only eat tack.
He sucks that which you have,
Leaving you with less than half.

Who am I?

Answer 1

You are

Death (or death's pale horse), among the Four Horsemen of the Apocalypse. In the book of Revelations, breaking seals will cause the horsemen to appear; hence the puzzle's title.

I come before the end, I am not your friend.

Referring to the apocalypse. Death (or its horse) is not your friend.

Brothers, have I three, They shall make you flee.

There are three other riders besides Death: Pestilence (sometimes Conquest), War, and Famine.

The first is white, And ready to fight. Posies in his hand, All are sick upon the land.

Pestilence (disease, all are sick) rides a white horse. There are also theories that posies were used to fend off plagues.

The second is red, Peace has fled. You call him to your aid, But his price must be paid.

War rides a red horse. There is no peace, so there is war.

The third is black, You can only eat tack. He sucks that which you have, Leaving you with less than half.

Famine rides a black horse. This stanza is referring to a food shortage.