Below the waves lies a world of unexplored depths and fearsome creatures; in the realms of fantasy, we can get even freakier with it! These tables are here to provide you with inspiration or random encounters to drop on your players and add some more life to the underwater environments in your worlds!

Sources

Each encounter focuses on a monster from throughout the D&D 5e library; to see what book or source they come from, you can consult the Book column. The sources unabbreviated sources are Basic Rules (BR), Monster Manual (MM), Mordenkainen Presents: Monsters of the Multiverse (MPMM), Icewind Dale: Rime of the Frostmaiden (RoF), Ghosts of Saltmarsh (GoS), Out of the Abyss (OoA), Volo’s Guide to Monsters (VGM), and Storm King’s Thunder (SKT).

Underground Random Encounters

The following table provides random encounters for parties playing in various “tiers” of gameplay; they generally reflect encounters you can sculpt to work for tables within a level range. The Low-tier can work for levels 1-4, Mid-tier for levels 5-11, and the Upper-tier for levels 12+.

Low-Tier Adventuering Random Encounter Table

            d12
            Book
            Encounter
            
            1
            BR
            A school of 1d6+4 quippers biting at anyone within their feeding area
            
            2
            MPMM
            1d4+2 dolphins eagerly hunting for fish
            
            3
            BR
            A scouting group of 1d4+3 merfolk hunting with two trained reef sharks
      
            4
            SKT
            1d6+2 sea elves and two giant sea horses traveling with nets full of goods
            
            5
            BR
            1d4+3 sahuagin searching through a shipwreck for food and valuables
            
            6
            BR
            A giant octopus and 1d4+1 quippers just vibing in a reef
            
            7
            MPMM
            1d4+1 sea spawn with different piscine anatomy lurking in flooded human ruins
            
            8
            BR
            A swarm of quippers with 1d4+1 loose quippers hunting together
            
            9
            BR
            A merrow hunter and its hunter shark companion with two kuo-tao captives
            
            10
            BR
            A sea hag dragging a recent victim to her submerged lair
            
            11
            GoS
            A shell shark and two following reef sharks seeking food
            
            12
            MM
            A sahuagin priestess and two sahuagin guards with a drowned sacrifice

Mid-Tier Adventuring Random Encounter Table

            d10
            Book
            Encounter
            
            1
            BR
            Two killer whales playing with the remains of a capsized boat
            
            2
            MPMM
            1d4+1 kraken priests on a pilgrimage to a sacred kraken birthing site
            
            3
            MM
            A sahuagin baron, two priestesses, and 1d4+1 sahuagin raiding party
      
            4
            BR
            A giant shark outfitted for battle with ships and its 1d6+2 merrow handlers
            
            5
            MPMM
            A sea hag prophecy coven defended by 2d6+2 merfolk cultists
            
            6
            BR
            A water elemental and 1d4+1 steam mephits destroying a merfolk settlement
            
            7
            GoS
            1d4+2 sahugin and a priestess summon a maw of sekolah to devour humanity
            
            8
            MM
            A disguised marid eager to bargain for property on the material plane
            
            9
            MPMM
            A morkoth compelling local kuo-tao into their lair for nourishment
            
            10
            MPMM
            1d4+3 deep scions in service to a plotting hyper-intelligent kraken

Upper-Tier Adventuring Random Encounter Table

            d8
            Book
            Encounter
            
            1
            GoS
            A juvenile kraken and 1d4+3 kraken priests raising it to destroy a city
            
            2
            SKT
            A storm giant bringing down a fleet of ships trespassing in their domain
            
            3
            MPMM
            Three wastriliths summoned to destroy a merfolk city
      
            4
            BR
            Two archdruids with two giant sharks and a killer whale seeking a rare object
            
            5
            MPMM
            A storm giant quintessent heralding a storm to punish a white dragon rival
            
            6
            BR
            A sleeping dragon turtle worshipped by a kobold society on a chain of islands
            
            7
            MPMM
            A leviathan awoken from ancient slumber thrashing out against civilization
            
            8
            BR
            A kraken determined to sink a nation of islands into the sea