I saw that a hydra has "advantage on saving throws against being... knocked unconscious" and was curious as to what spells or effects would cause such a thing to happen. The only effect that came to mind is the sleep spell, which operates on hit points rather than a saving throw. What else can knock a creature unconscious?
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\$\begingroup\$ For your purposes, do you include being reduced to zero hit points as a cause of unconsciousness? \$\endgroup\$– Mark WellsCommented Oct 19, 2020 at 23:07
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\$\begingroup\$ Being reduced to zero hit points, as far as I know, bypasses any "save or be unconscious" mechanics by typically forcing the condition, and so I do not include this in the question. (Abilities that let a creature perform some kind of save to remain at 1 hit point automatically fail the 0 hit point requirement, since they avoid going to 0 hit points entirely.) If there is something here that I missed, I'd be glad to hear it. \$\endgroup\$– Saber CrusaderCommented Oct 22, 2020 at 23:59
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Spells
Fortunately D&D Beyond categorizes spells by applied conditions and lets you filter by it. Doing so lists eyebite, which applies unconscious as part of its Asleep mode, and symbol, which does so as part of its Sleep mode. In addition the spells sleep and catnap (not listed above) apply the condition, but not contingent on saving throws.
Whether the Slumber mode of imprisonment counts is unclear, the spell does not call out the unconscious condition, but asleep is often taken to mean unconscious. Ask your DM before making a sleeping beauty hydra.
Monster abilities
In addition a number of monster traits cause the unconscious condition contingent on a saving throw, such as the chasme's Drone:
Any other creature that starts its turn within 30 feet of the chasme must succeed on a DC 12 Constitution saving throw or fall unconscious for 10 minutes. [...]
There's also the Ogre Chain Brute's Chain Smash, the Kamadan's Sleep Breath, the Beholder's (and Gauth's and Death Tyrant's) Sleep ray, the Gentle Lullaby of a Satyr with pipes, and the Sleep Breath of Brass Dragons (and probably many others). Effects which cause the unconscious condition contingent on a poisoning (usually by failing the saving throw by 5 or more), such as the pseudodragon's Sting presumably aren't valid.
To my knowledge there are no player character options which include the effect (outside the above spells, or summoning an applicable monster, nor any magic items (where it isn't part of a poison effect), but I welcome any additions I've missed.
answered Oct 19, 2020 at 22:09