What Evil Lurks...What Good can battle it?

Why does the Alter Rebbe open up Iggeret HaTshuva with the word Tanya? This word is not found in the source he is quoting in Tractate Yoma, nor is it found in most sources that quote this particular teaching about the Three Forms of Atonement.

Rather, the Alter Rebbe is trying to connect between the first section of Tanya and this section, Iggeret HaTeshuva, by emphasizing their role in eradicating a certain very negative spiritual affliction prevalent amongst Torah Scholars called Tanya. Why is this work so focused against this one particular affliction?

In Derech Chaim of the Mitteler Rebbe, it's explained that the deepest evil is the type that lies dormant and unrevealed in the soul. This is the most dangerous evil of all.  If I don't see it then I can't fix it. We usually identify evil through its manifestation in thought, speech and action. However, Chassidus teaches us that the potential for evil is also a form of evil. In fact, it is the most unpredictable and hardest to eradicate form of evil. Even a complete tzaddik has to fear from this type of evil as in the famous story where the Mitteler Rebbe stopped a private meeting with one of his chassidim and fasted and repented for three days since he couldn't find this chassid's sin in himself, and feared it was due to a form of the Depth of Evil.

The Depth of Evil is most prevalent amongst Torah Scholars as they also have evil in them but learn to subdue it through Torah learning and ego-nullification. So it is by Torah Scholars that this shell, nicknamed Tanya, can come to rule.

The work of the Alter Rebbe,Tanya, is the ultimate fixing for this state of covering since it has the power to illuminate and unearth the essence of the soul - the nucleus of the soul, and with it all the other levels as well. When this level of the soul is uncovered, it is impossible for evil to remain covered. This is also why the letters of Tanya re-scrambled spell out Eitan, or Enduring Strength, representing the eternal endurance of this level of the Jewish soul, also known as Yechida.