The Unsealed letter is an item used in The Fremennik Exiles quest. It is obtained by killing a Basilisk Youngling that comes out after the player searches the sandpit in Rellekka. It is given to Freygerd along with the fang and venom gland as evidence of basilisks plotting to attack Rellekka.
For years we have waited and at last, our time to rise has come. Soon, we will bring terror to the Fremennik Province. Go forth brothers and sisters. Go forth and wreak havoc amongst their people. Once they are distracted, we will tear down the walls that hold our king. We will finally free the Jormungand! The time is ours. They will be unprepared. Their ban on magic shall be their downfall. They have no champion to defend them this time. No V.
For an exhaustive list of all known sources for this item, see here