The caster does a ritual that makes all creatures "forget" about the target creature. This spans across all planes, and affects all creatures that are lower level than the spell caster. if someone is the same level as the caster then they must roll an INT saving throw. if a creature is higher level than the caster they get the amount of levels they are higher than the caster times 2 as a bonus to the roll. if a creature is obscenely powerful (I.E a god/powerful warlock patron/aboleth) the DM chooses whether or not it rolls. the caster can choose whether or not they forget the target of the spell. any, and all written documents/paintings/oral traditions that contain the target are magically altered by the spell in such a way that they no longer include the target. if the target of the spell is a much higher level/obscenely powerful being it is up to the DM whether or not any creatures roll. if a dead creature/inanimate object/piece of knowledge that is not used often is targeted by the spell then it is not rolled for, simply forgotten. If a creature does not know about the targets existence then they can not learn about it unless told by someone who did not forget the target.