Some spells, feats and abilities that got cut from Tasha’s original manuscript of Tasha’s Guide to the Planes when it was published as Tasha’s Cauldron of Everything.

Some because they were considered ‘out of style’ or ‘too niche’, and others because they were deemed ‘too dangerous’ to have in common use.

SPELL – Know Alignment – Level 2 – Cleric, Druid – Lost Spell from 2nd edition that can be learned from Koran, Tasha the Dark or acquired via Feats or Magical Secrets.

Spell – Summon Monster I-IX – Level 1-9 – Wizard, Druid, Warlock – Lost spell that later evolved into the Conjure and Summon spells of 5th edition.

Tasha the Dark had been working on a version of this spell having long been annoyed of the lack of a ‘all in one handy dandy summoning spell’ but her attempts to publish it have failed due to the sheer danger it presented to the Caster.
As a result this spell can not be learned by magical secrets.

It’s possible Tasha herself knows how to cast this and that it may be found in the original manuscript of Tasha’s Guide to the Planes.