Need help thinking of an effect for a magic (level 20) gem.

Elijah V'enditaben, Mikko 'anken, Sierra T'kan, Oscar and Theseus! If you're reading this, please don't.

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I am running a pirate themed camapaign, and the bbeg is a kraken. Here's my plan for how it comes about, I am drawing a blank on what i want the Gem of the Fiend to do. Feel free to think of ideas for the other gems too! so far theyre just re-skins of spells (Prayer of Healing, Create Undead, Mass Heal)

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There are parts of an artefact that once reassembled, will summon a kraken.

Artefact pieces

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Level 5

Gem of the living:

Casting time: 1 action | Duration: Instantaneous | Components: V, S | Range: 30 ft

Up to three creatures of your choice that you can see within range each regain hit points equal to 2d8 + your proficiency bonus. This spell has no effect on undead or constructs.

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Level 10

Gem of the Undead

Casting time: 1 action | Duration: Instantaneous | Components: V, S | Range: 10 ft

Choose up to three corpses of Medium or Small humanoids within range. Each corpse becomes a ghoul under your control. As a bonus action on each of your turns, you can mentally command any creature you animated with this spell if the creature is within 120 feet of you (if you control multiple creatures, you can command any or all of them at the same time, issuing the same command to each one). You decide what action the creature will take and where it will move during its next turn, or you can issue a general command, such as to guard a particular chamber or corridor. If you issue no commands, the creature only defends itself against hostile creatures. Once given an order, the creature continues to follow it until its task is complete. The creature is under your control for 1 hour, after which it turns to ash, dropping any gear it has on it.

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Level 15

Gem of the Celestial

Casting time: 1 action | Duration: Instantaneous | Components: V, S | Range: 60 ft

A flood of healing energy flows from you into injured creatures around you. You restore up to 500 hit points, divided as you choose among any number of creatures that you can see within range. Creatures healed by this spell are also cured of all diseases and any effect making them blinded or deafened. This spell has no effect on undead or constructs.

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Level 20

Gem of the Fiend

?????