|Well, congrats. You discovered it. You get to name it.|
Although this article is based on canonical information, the actual name of this subject is conjectural.
According to Men of Letters's "Infamati et obliterati" (Dishonored and Forgotten) members files, in 1954 (2 years before his "formal separation" in 1956), he found (or maybe invented) this spell and proposed the usage of this spell to render the Men of Letters bunker unplottable and only accessible by a special countering spell. His proposal to use the spell was rejected by Men of Letters and he was kicked out of the organization in 1956. However, that wasn't the end of the spell. He used this spell to render his own mansion invisible and unplottable.
Not much known about the spell. The procedure of this spell has never be shown. However, once cast, the spell heavily warded the place where the spell was cast and the place became invisible and unplottable. The warded place became unplottable for an indefinite amount of time with the effect persisting even after the death of the caster.
The only known way to counter the spell is with the Unplottable Place Accessing Spell cast at the entrance of the warded place.