Creepiest D&D Content

[Lapak]

Neogi usually get my vote for creepy factor. Particularly their reproduction cycle.

Since you remind me of it, yes, that is one of the creepier things D&D has produced. And in a setting (Spelljammer) that wasn't overtly creepy most of the time. But alongside matchlock-wielding hippo mercenaries and Giant Space Hamsters that literally power spaceships by running on exercise wheels, you have a race of ruthless hermaphroditic spider-monster slavers. Who reproduce by waiting until one of their kind starts to suffer from the senility of old age, then subject it to a (thankfully not detailed) hormonal/chemical/letsnotconsiderittooclosely process that causes it to go completely mindless and swell up with young who are birthed by devouring it from the inside.

Which, just as a side effect, leads to encounters with ships captained by aging monsters who already had no conscience and are now suffering from both dementia and an ironclad determination to avoid the notice of their kindred at any costs.