The Necromancy of Thay is a powerful book of the dead that you may discover in Act I but you won’t be able to fully unlock its potential yet until you reach Act III.
A quest called Search the Cellar remains until later and a sub-quest called Unlock the Ancient Tome will be updated as you progress during the act.
Mastering the Necromancy of Thay requires that you go through a simple process of finding a missing book, which will allow you to read it to unlock its true power.
How To Master The Necromancy of Thay?
Once you make it to Act III, you will discover that the Necromancy of Thay can be unlocked but only if you acquire knowledge from a book called The Tharchiate Codex.
This information is revealed by speaking to Tolna at the Sorcerous Sundries or Mystic Carrion at Philgrave’s Mansion.
You may later obtain The Tharchiate Codex by heading to the Sorcerous Vault and entering the Elminster Vault, where it is locked in a mundane Display Case. (You can pick the lock)
You will need to read The Tharchiate Codex and afterward, reading the Necromancy of Thay will allow you to unlock its full potential after a few Wisdom Checks.
Necromancy of Thay Unlock Rewards
Once you have read the Necromancy of Thay after reading The Tharchiate Codex, you will gain the Danse Macabre spell.
This allows you to summon 4 Ghouls that will fight alongside you (they are uncontrollable and act on their own) with a chance to paralyze their targets. (Once per Long Rest)
Tharchiate Withering
Tharchiate Withering is a curse that the person who reads The Tharchiate Codex receives, which reduces their constitution score by 5.
This may be removed by using spells such as Remove Curse or Greater Restoration and will cause the person who had it removed to gain Tharchiate Vigour.
Tharchiate Vigor is a permanent passive that grants you 20 Temporary Hitpoints every time you take a Long Rest.
Verdict
The Tharchiate Codex is not that hard to obtain if you know how to get to the Sorcerous Vault, but this also means you need to first get to the Lower City.
Unlocking the spells from the Necromancy of Thay can be useful in certain situations, especially when you master the Tome since this allows you to summon Ghouls to help you.
Tharchiate Vigor can be a drastic change for any character, giving them more survivability with the additional hit points they have after each long rest.