Elder Sign Domain

The Elder Sign domain represents a deity’s recognition of the dark powers looming beyond the known realities and of forces beyond the known planes of existence. Most clerics who use the Elder Sign domain belong to obscure sects or tenuously connected orders, mistrusted or outright persecuted by the main body of a religion. Elder Sign priests discover terrible lore and must ask themselves whether preserving that lore is worth risking the dangers inherent in such endeavors. As a cleric with the Elder Sign domain you can invoke the ancient Elder Sign to ward off the Outer Gods and their servants. 

 

Note: Manually add Elder Sign as a spell to your Domain Spells at 5th Level. Further: This is a subclass from Sandy Peterson's Cthulhu Mythos 5E Book.

Elder Knowledge

When you choose this domain you gain proficiency in Arcana and History

Textual Expertise

Starting at 1st level, you gain insight regarding magic traps, spellbooks, Mythos texts, and spells that create glyphs, signs, symbols, or other magical writing. You add twice your proficiency bonus instead of your normal proficiency bonus on Intelligence (Arcana) and Wisdom (Yog-Sothothery) checks to notice, identify, or research magic traps and texts if you are proficient with those skills.

Channel Divinity: Detect Otherworldly Influence

As an action, you present your holy symbol and call upon your deity’s wisdom to expand your senses in your immediate vicinity. For the next 10 minutes, you know if there is an aberration, celestial, elemental, fey, fiend, monstrosity, or undead within 30 feet of you and which types are present (although not how many or where they are).
You also know if there is a Mythos text or Elder Influence within range. As an action during the duration, you can concentrate on a creature you can see to learn which of those types it is, if any.

Channel Divinity: Forbid Knowledge

As an action, you present your holy symbol and evoke scouring divine energy to destroy all copies of a chosen piece of information in a 20-foot cube adjacent to you. Within the area, brief nonmagical records of the information become blank without affecting surrounding text. Detailed nonmagical records turn to ash. Magical records aren’t affected. Any creature who knows the information must succeed on an Intelligence saving throw against your spell save DC or forget the information until greater restoration, remove curse, or similar magic is cast upon the creature. On a failed save, the creature can make one new saving throw to recall the information when it next finishes a long rest, but if it fails again, the information remains forgotten indefinitely. If forgotten knowledge caused dread or insanity, those conditions have no effect on the creature until the forgotten knowledge is recalled. If pressed on the topic, the creature is aware it is forgetting something; the creature does not know the exact nature of the information or that you erased the memory.

Unmaking Spells

At 8th level, your spells become more potent. Whenever you cast a spell that deals damage, you deal an additional 1d6 force damage to one target. At 14th level, this damage increases to 2d6. You can only deal this additional damage once per turn. Additionally, if you cast dispel magic and must make an ability check for the spell to succeed, you have advantage on the check if you are targeting a magic trap, a spell that created magical writing, or a spell cast by an aberration, celestial, elemental, fey, fiend, or undead.

Channel Divinity: Seal the Immortal

At 17th level, you choose four spells from the wizard spell list, one from each of the following levels: 6th, 7th, 8th, and 9th. You add them to your list of domain spells. Like your other domain spells, they are always prepared and count as cleric spells for you.