Elder Sign

You create an Elder Sign on an object you touch. The Elder Sign is immune to the damage, spells, and abilities from any Elder Influence, or creatures bound to their service (including deep ones or Tcho-Tcho, and most aberrations). These creatures can't touch or pass through an object with the Elder Sign. Generally, an Elder Sign is destroyed after being within 30 feet of an Elder Influence for more than 5 continuous rounds. Note that since only the Elder Sign drawn on it would not be affected by a Great-Old One's attack, but the monster could still strike the bearer if it avoids striking the shield (and thus the bearer's AC is not any higher than it would be with an unmarked shield). Engraving the sign upon a door or on an archway prevents such creatures from passing through the portal.

You can create an Elder Sign larger than 5 feet across when you cast this spell as a formula by increasing the ability check DCs by 1 each per 5 feet per level of the spell slot above 3rd.