Your search for "*monster of the abyss*" revealed the following:

Aclazotz, Deepest Betrayal // Temple of the Dead
[The Lost Caverns of Ixalan]

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Guardian Dreadscythe
[DLCS-EN012 Common]

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Tour Bus From the Underworld
[BP02-EN105 Common]

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Rust Monster (Showcase)
[Dungeons & Dragons: Adventures in the Forgotten Realms]

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Debris Dragon
[AP01-EN002 Ultimate Rare]

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Rust Monster
[The List]

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Illuna, Apex of Wishes - Ghidorah, King of the Cosmos (Godzilla Series)
[Ikoria: Lair of Behemoths]

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Abyssal Harvester (Extended Art)
[Foundations]

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Token: Yusei
[TKN5-EN013 Super Rare]

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Token: Yuya
[TKN5-EN015 Super Rare]

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Belial - Marquis of Darkness
[BP02-EN061 Rare]

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Monstrosity of the Lake
[The Lord of the Rings: Tales of Middle-Earth Commander]

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Goblin Elite Attack Force
[BP02-EN040 Mosaic Rare]

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Necro Gardna
[BP02-EN052 Mosaic Rare]

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Invader of Darkness
[BPW2-EN010 Common]

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Belial - Marquis of Darkness
[BP02-EN061 Mosaic Rare]

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