The Dark Libram of the Abyss

This ancient book tells of a secret blood cult aimed at overthrowing order and conquering Otherwhere by stealing souls and corrupting them into an army of mutant demons. The scheme was thwarted when four mystic heroes referred to as "Martyrs of the Black Blade" told the gods of judging and guiding the dead where such spirits had been hidden.

Some page spreads in this book cause a demonic face to emerge and begin bargaining for the reader's soul, but such manifestations have no power to compel or fulfill any actual powers.

Among the text are pages that function as a spell scroll...

Summon Greater Demon
4th-level conjuration

Casting Time: 1 action
Range: 60 feet
Duration: Concentration, up to 1 hour

You utter foul words, summoning one demon from the chaos of the Abyss. You choose the demon's type, which must be one of challenge rating 5 or lower, such as a shadow demon or a barlgura. The demon appears in an unoccupied space you can see within range, and the demon disappears when it drops to 0 hit points or when the spell ends.

Roll initiative for the demon, which has its own turns. When you summon it and on each of your turns thereafter, you can issue a verbal command to it (requiring no action on your part), telling it what it must do on its next turn. If you issue no command, it spends its turn attacking any creature within reach that has attacked it.

At the end of each of the demon's turns, it makes a DC 14 Charisma saving throw. The demon has disadvantage on this saving throw if you say its true name. On a failed save, the demon continues to obey you. On a successful save, your control of the demon ends for the rest of the duration, and the demon spends its turns pursuing and attacking the nearest non-demons to the best of its ability. If you stop concentrating on the spell before it reaches its full duration, an uncontrolled demon doesn't disappear for 1d6 rounds if it still has hit points.

As part of casting the spell, you can form a circle on the ground with the blood used as a material component. The circle is large enough to encompass your space. While the spell lasts, the summoned demon can't cross the circle or harm it, and it can't target anyone within it. Using the material component in this manner consumes it when the spell ends.

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SPELL SCROLL RULES (DMG p. 200)

When using a spell scroll, the spell must be on a caster's class spell list to use as an action (no need to provide any spell's components). Otherwise, the scroll is unintelligible.

If the spell is on a character's class spell list but of a higher level than can normally be cast, the character must make an ability check using the usual class-related spellcasting ability (Intelligence, Wisdom, or Charisma) to determine whether it is cast successfully. The DC equals 10 + spell's level. On a failed check, the spell disappears from the scroll with no other effect.

REQUIRED SPELLCASTING ABILITY CHECK
* Cantrip (Common) ... DC 10
* 1st Level Spell (Common) ... DC 11
* 2nd Level Spell (Uncommon) ... DC 12
* 3rd Level Spell (Uncommon) ... DC 13
* 4th Level Spell (Rare) ... DC 14
* 5th Level Spell (Rare) ... DC 15
* 6th Level Spell (Very rare) ... DC 16
* 7th Level Spell (Very rare) ... DC 17
* 8th Level Spell (Very rare) ... DC 18
* 9th Level Spell (Legendary) ... DC 19

Once the spell is cast, the words on the scroll fade, and those pages within the book fall out as dust.