mentioned above, which appears in 5e in Mordenkainen's Tome of Foes (since it is associated with water, though the book mentions no relation to Dagon specifically, but that Forgotten Realms Wiki link does), but it's CR is quite high for the party level range I'm planning. There...
FFiigure 5 is the top-level view of the test apparatus. IItt sshhoowwss tthhee ssoouurrce oof ssweep voltage and the mmeasurement of net voltage (sweep voltage plus any contribution by the ATTEECC)) aaccrroosssstthheeAATTEECC.. FFiigguurree 55.. AATTEECC wwiitthh sswweeeeppiinngg vvoo...